Methods for driving electro-optic displays

ABSTRACT

An electro-optic display, having at least one pixel capable of achieving any one of at least four different gray levels including two extreme optical states, is driven by displaying a first image on the display, and rewriting the display to display a second image thereon, wherein, during the rewriting of the display, any pixel which has undergone a number of transitions exceeding a predetermined value without touching an extreme optical state, is driven to at least one extreme optical state before driving that pixel to its final optical state in the second image.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/879,335,filed Jun. 24, 2004 (Publication No. 2005/0024353, now U.S. Pat. No.7,528,822), which claims benefit of the following ProvisionalApplications: (a) Ser. No. 60/481,040, filed Jun. 30, 2003; (b) Ser. No.60/481,053, filed Jul. 2, 2003; and (c) Ser. No. 60/481,405, filed Sep.22, 2003.

The aforementioned application Ser. No. 10/879,335 is also acontinuation-in-part of application Ser. No. 10/814,205, filed Mar. 31,2004 (Publication No. 2005/0001812, now U.S. Pat. No. 7,119,772), whichitself claims benefit of the following Provisional Applications: (d)Ser. No. 60/320,070, filed Mar. 31, 2003; (e) Ser. No. 60/320,207, filedMay 5, 2003; (f) Ser. No. 60/481,669, filed Nov. 19, 2003; (g) Ser. No.60/481,675, filed Nov. 20, 2003; and (h) Ser. No. 60/557,094, filed Mar.26, 2004.

The aforementioned copending application Ser. No. 10/814,205 is also acontinuation-in-part of application Ser. No. 10/065,795, filed Nov. 20,2002 (Publication No. 2003/0137521, now U.S. Pat. No. 7,012,600), whichitself claims benefit of the following Provisional Applications: (i)Ser. No. 60/319,007, filed Nov. 20, 2001; (j) Ser. No. 60/319,010, filedNov. 21, 2001; (k) Ser. No. 60/319,034, filed Dec. 18, 2001; (l) Ser.No. 60/319,037, filed Dec. 20, 2001; and (m) Ser. No. 60/319,040, filedDec. 21, 2001.

This application is also related to application Ser. No. 10/249,973,filed May 23, 2003 (now U.S. Pat. No. 7,193,625), which is acontinuation-in-part of the aforementioned application Ser. No.10/065,795. Application Ser. No. 10/249,973 claims priority fromProvisional Application Ser. No. 60/319,315, filed Jun. 13, 2002 andSer. No. 60/319,321, filed Jun. 18, 2002. This application is alsorelated to application Ser. No. 10/063,236, filed Apr. 2, 2002(Publication No. 2002/0180687, now U.S. Pat. No. 7,170,670).

The entire contents of these copending applications, and of all otherU.S. patents and published and copending applications mentioned below,are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to methods for driving electro-optic displays.The methods of the present invention are especially, though notexclusively, intended for use in driving bistable electrophoreticdisplays.

The term “electro-optic” as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the patentsand published applications referred to below describe electrophoreticdisplays in which the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned the transition between the two extreme states may notbe a color change at all.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the imaging art to refer to displays comprisingdisplay elements having first and second display states differing in atleast one optical property, and such that after any given element hasbeen driven, by means of an addressing pulse of finite duration, toassume either its first or second display state, after the addressingpulse has terminated, that state will persist for at least severaltimes, for example at least four times, the minimum duration of theaddressing pulse required to change the state of the display element. Itis shown in published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

The term “impulse” is used herein in its conventional meaning in theimaging art of the integral of voltage with respect to time. However,some bistable electro-optic media act as charge transducers, and withsuch media an alternative definition of impulse, namely the integral ofcurrent over time (which is equal to the total charge applied) may beused. The appropriate definition of impulse should be used, depending onwhether the medium acts as a voltage-time impulse transducer or a chargeimpulse transducer.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071; 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in U.S. Patent Application 2003/0214695. This type ofmedium is also typically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,724,519; and 6,727,881; and U.S.Patent Applications Publication Nos. 2002/0019081; 2002/0021270;2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980;2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792;2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560;2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755;2003/0053189; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717;2003/0151702; 2003/0189749; 2003/0214695; 2003/0214697; 2003/0222315;2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634;and 2004/0094422; and International Applications Publication Nos. WO99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/092077; WO03/107315; WO 2004/017135; and WO 2004/023202.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called “polymer-dispersed electrophoretic display” inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withincapsules but instead are retained within a plurality of cavities formedwithin a carrier medium, typically a polymeric film. See, for example,International Application Publication No. WO 02/01281, and U.S. PatentApplication Publication No. 2002/0075556, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346. Other types ofelectro-optic displays may also be capable of operating in shutter mode.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior(such displays may hereinafter for convenience be referred to as“impulse driven displays”), is in marked contrast to that ofconventional liquid crystal (“LC”) displays. Twisted nematic liquidcrystals act are not bi- or multi-stable but act as voltage transducers,so that applying a given electric field to a pixel of such a displayproduces a specific gray level at the pixel, regardless of the graylevel previously present at the pixel. Furthermore, LC displays are onlydriven in one direction (from non-transmissive or “dark” to transmissiveor “light”), the reverse transition from a lighter state to a darker onebeing effected by reducing or eliminating the electric field. Finally,the gray level of a pixel of an LC display is not sensitive to thepolarity of the electric field, only to its magnitude, and indeed fortechnical reasons commercial LC displays usually reverse the polarity ofthe driving field at frequent intervals. In contrast, bistableelectro-optic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Whether or not the electro-optic medium used is bistable, to obtain ahigh-resolution display, individual pixels of a display must beaddressable without interference from adjacent pixels. One way toachieve this objective is to provide an array of non-linear elements,such as transistors or diodes, with at least one non-linear elementassociated with each pixel, to produce an “active matrix” display. Anaddressing or pixel electrode, which addresses one pixel, is connectedto an appropriate voltage source through the associated non-linearelement. Typically, when the non-linear element is a transistor, thepixel electrode is connected to the drain of the transistor, and thisarrangement will be assumed in the following description, although it isessentially arbitrary and the pixel electrode could be connected to thesource of the transistor. Conventionally, in high resolution arrays, thepixels are arranged in a two-dimensional array of rows and columns, suchthat any specific pixel is uniquely defined by the intersection of onespecified row and one specified column. The sources of all thetransistors in each column are connected to a single column electrode,while the gates of all the transistors in each row are connected to asingle row electrode; again the assignment of sources to rows and gatesto columns is conventional but essentially arbitrary, and could bereversed if desired. The row electrodes are connected to a row driver,which essentially ensures that at any given moment only one row isselected, i.e., that there is applied to the selected row electrode avoltage such as to ensure that all the transistors in the selected roware conductive, while there is applied to all other rows a voltage suchas to ensure that all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed to that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner.

It might at first appear that the ideal method for addressing such animpulse-driven electro-optic display would be so-called “generalgrayscale image flow” in which a controller arranges each writing of animage so that each pixel transitions directly from its initial graylevel to its final gray level. However, inevitably there is some errorin writing images on an impulse-driven display. Some such errorsencountered in practice include:

(a) Prior State Dependence; With at least some electro-optic media, theimpulse required to switch a pixel to a new optical state depends notonly on the current and desired optical state, but also on the previousoptical states of the pixel.

(b) Dwell Time Dependence; With at least some electro-optic media, theimpulse required to switch a pixel to a new optical state depends on thetime that the pixel has spent in its various optical states. The precisenature of this dependence is not well understood, but in general, moreimpulse is required that longer the pixel has been in its currentoptical state.

(c) Temperature Dependence; The impulse required to switch a pixel to anew optical state depends heavily on temperature.

(d) Humidity Dependence; The impulse required to switch a pixel to a newoptical state depends, with at least some types of electro-optic media,on the ambient humidity.

(e) Mechanical Uniformity; The impulse required to switch a pixel to anew optical state may be affected by mechanical variations in thedisplay, for example variations in the thickness of an electro-opticmedium or an associated lamination adhesive. Other types of mechanicalnon-uniformity may arise from inevitable variations between differentmanufacturing batches of medium, manufacturing tolerances and materialsvariations.

(f) Voltage Errors; The actual impulse applied to a pixel willinevitably differ slightly from that theoretically applied because ofunavoidable slight errors in the voltages delivered by drivers.

General grayscale image flow suffers from an “accumulation of errors”phenomenon. For example, imagine that temperature dependence results ina 0.2 L* (where L* has the usual CIE definition:L*=116(R/R0)⅓−16,where R is the reflectance and R0 is a standard reflectance value) errorin the positive direction on each transition. After fifty transitions,this error will accumulate to 10 L*. Perhaps more realistically, supposethat the average error on each transition, expressed in terms of thedifference between the theoretical and the actual reflectance of thedisplay is ±0.2 L*. After 100 successive transitions, the pixels willdisplay an average deviation from their expected state of 2 L*; suchdeviations are apparent to the average observer on certain types ofimages.

This accumulation of errors phenomenon applies not only to errors due totemperature, but also to errors of all the types listed above. Asdescribed in the aforementioned 2003/0137521, compensating for sucherrors is possible, but only to a limited degree of precision. Forexample, temperature errors can be compensated by using a temperaturesensor and a lookup table, but the temperature sensor has a limitedresolution and may read a temperature slightly different from that ofthe electro-optic medium. Similarly, prior state dependence can becompensated by storing the prior states and using a multi-dimensionaltransition matrix, but controller memory limits the number of statesthat can be recorded and the size of the transition matrix that can bestored, placing a limit on the precision of this type of compensation.

Thus, general grayscale image flow requires very precise control ofapplied impulse to give good results, and empirically it has been foundthat, in the present state of the technology of electro-optic displays,general grayscale image flow is infeasible in a commercial display.

Almost all electro-optic medium have a built-in resetting (errorlimiting) mechanism, namely their extreme (typically black and white)optical states, which function as “optical rails”. After a specificimpulse has been applied to a pixel of an electro-optic display, thatpixel cannot get any whiter (or blacker). For example, in anencapsulated electrophoretic display, after a specific impulse has beenapplied, all the electrophoretic particles are forced against oneanother or against the capsule wall, and cannot move further, thusproducing a limiting optical state or optical rail. Because there is adistribution of electrophoretic particle sizes and charges in such amedium, some particles hit the rails before others, creating a “softrails” phenomenon, whereby the impulse precision required is reducedwhen the final optical state of a transition approaches the extremeblack and white states, whereas the optical precision required increasesdramatically in transitions ending near the middle of the optical rangeof the pixel.

Various types of drive schemes for electro-optic displays are knownwhich take advantage of optical rails. For example, FIGS. 9 and 10 ofthe aforementioned 2003/0137521 (reproduced below), and the relateddescription at Paragraphs [0177] to [0180], describe a “slide show”drive scheme in which the entire display is driven to both optical railsbefore any new image is written. Such a slide show drive scheme producesaccurate grayscale levels, but the flashing of the display as it isdriven to the optical rails is distracting to the viewer. It has alsobeen suggested (see the aforementioned U.S. Pat. No. 6,531,997) that asimilar drive scheme be employed in which only the pixels, whose opticalstates need to be changed in the new image, be driven to the opticalrails. However, this type of “limited slide show” drive scheme is, ifanything, even more distracting to the viewer, since the solid flashingof a normal slide show drive scheme is replaced by image dependentflashing, in which features of the old image and the new image flash inreverse color on the screen before the new image is written.

Obviously, a pure general grayscale image flow drive scheme cannot relyupon using the optical rails to prevent errors in gray levels since insuch a drive scheme any given pixel can undergo an infinitely largenumber of changes in gray level without ever touching either opticalrail.

In one aspect, this invention seeks to provide methods for achievingcontrol of gray levels in electro-optic displays which achieve stabilityof gray levels similar to those achieved by slide show drive schemes butwhich do not suffer from the distracting flashing of slide show driveschemes. Preferred methods of the present invention can give the viewera visual experience similar to that provided by a pure general grayscaleimage flow drive scheme.

In another aspect, this invention seeks to provide methods for achievingfine control of gray levels in displays driven by pulse widthmodulation.

When driving an active matrix display having a bistable electro-opticmedium to write gray scale images thereon, it is desirable to be able toapply a precise amount of impulse to each pixel, so as to achieveaccurate control of the gray scale displayed. The driving method usedmay rely modulation of the voltage applied to each pixel and/ormodulation of the “width” (duration) for which the voltage is applied.Since voltage modulated drivers and their associated power supplies arerelatively costly, pulse width modulation is commercially attractive.However, during the scanning of an active matrix display using suchpulse width modulation, conventional driver circuitry only allows one toapply a single voltage to any given pixel during any one scan of thematrix. Consequently, pulse width modulation driving of active matrixdisplays is effected by scanning the matrix multiple times, with thedrive voltage being applied during none, some or all of the scans,depending upon the change desired in the gray level of the specificpixel. Each scan may be regarded as a frame of the drive waveform, withthe complete addressing pulse being a superframe formed by a pluralityof successive frames. It should be noted that, although the drivevoltage is only applied to any specific pixel electrode for one lineaddress time during each scan, the drive voltage persists on the pixelelectrodes during the time between successive selections of the sameline, only slowly decaying, so that the pixel is driven betweensuccessive selections of the same line.

As already mentioned, each row of the matrix needs to be individuallyselected during each frame so that for high resolution displays (forexample, 800×600 pixel displays) in practice the frame rate cannotexceed about 50 to 100 Hz; thus each frame typically lasts 10 to 20 ms.Frames of this length lead to difficulties in fine control of gray scalewith many fast switching electro-optic medium. For example, someencapsulated electrophoretic media substantially complete a switchbetween their extreme optical states (a transition of about 30 L* units)within about 100 ms, and with such a medium a 20 ms frame corresponds toa gray scale shift of about 6 L* units. Such a shift is too large foraccurate control of gray scale; the human eye is sensitive todifferences in gray levels of about 1 L* unit, and controlling theimpulse only in graduations equivalent to about 6 L* units is likely togive rise to visible artifacts, such as “ghosting” due to prior statedependence of the electro-optic medium, and pulses needed to ensure thatthe waveform used is DC balanced (see the applications mentioned in the“Cross Reference to Related Applications” section above). Morespecifically, ghosting may be experienced because, as discussed in someof the aforementioned patents and applications, the variation of graylevel with applied impulse is not linear, and the total impulse neededfor any specific change in gray level may vary with the time at whichthe impulse is applied and the intervening gray levels. For example, ina simple 4 gray level (2 bit) display having gray levels 0 (black), 1(dark gray), 2 (light gray) and 3 (white), driven by a simple pulsewidth modulation drive scheme, these non-linearities may result in theactual gray level achieved after a notional 0-2 transition beingdifferent from the gray level achieved after a notional 1-2 transition,with the production of highly undesirable visual artifacts. Thisinvention provides methods for achieving fine control of gray levels indisplays driven by pulse width modulation, thus avoiding theaforementioned problems.

SUMMARY OF INVENTION

Accordingly, in one aspect, this invention provides a method for drivingan electro-optic display having at least one pixel capable of achievingany one of at least four different gray levels including two extremeoptical states. The method comprises:

displaying a first image on the display; and

rewriting the display to display a second image thereon, wherein, duringthe rewriting of the display any pixel which has undergone a number oftransitions exceeding a predetermined value, the predetermined valuebeing at least one, without touching an extreme optical state, is drivento at least one extreme optical state before driving that pixel to itsfinal optical state in the second image.

This method may hereinafter for convenience be referred to as the“limited transitions method” of the present invention.

In one form of this limited transitions method, the rewriting of thedisplay is effected such that, once a pixel has been driven from oneextreme optical state towards the opposed extreme optical state by apulse of one polarity, the pixel does not receive a pulse of the opposedpolarity until it has reached the opposed extreme optical state.

Also, in the limited transitions methods, the predetermined value(predetermined number of transitions) is not greater than N/2, where Nis the total number of gray levels capable of being displayed by apixel. The limited transitions method may be effected using a tri-leveldriver, i.e., the rewriting of the display may be effected by applyingto the or each pixel any one or more of voltages −V, 0 and +V. Thelimited transitions method may also be DC-balanced, i.e., the rewritingof the display may be effected such that, for any series of transitionsundergone by a pixel, the integral of the applied voltage with time isbounded.

In the limited transitions method of the present invention, therewriting of the display may be effected such that the impulse appliedto a pixel during a transition depends only upon the initial and finalgray levels of that transition. Alternatively, the method may be adaptedto take account of other states of the display, as described in moredetail below. In one preferred form of the limited transitions method,for at least one transition undergone by the at least one pixel from agray level R2 to a gray level R1, there is applied to the pixel asequence of impulses of the form:

-   -   −TM(R1,R2) IP(R1)-IP(R2) TM(R1,R2)        where “IP(Rx)” represents the relevant value from an impulse        potential matrix having one value for each gray level, and        TM(R1,R2) represents the relevant value from a transition matrix        having one value for each R1/R2 combination. (For convenience,        impulse sequences of this type may hereinafter be abbreviated as        “−x/ΔIP/x” sequences.) Such −x/ΔIP/x sequences may be used for        all transitions in which the initial and final gray levels are        different. Also, in such −x/ΔIP/x sequences, the final “x”        section may occupy more than one half of the maximum update        time. The TM(R1,R2) or x values may be chosen such that the sign        of each value is dependent only upon R1; in particular, these        values may be chosen to be positive for one or more light gray        levels and negative for one or more dark gray levels so that        gray levels other than the two extreme optical states are        approached from the direction of the nearer extreme optical        state.

The aforementioned −x/ΔIP/x sequences may contain additional pulses. Inparticular, such sequences may comprise an additional pair of pulses ofthe form [+y][−y], where y is an impulse value, which may be eithernegative or positive, the [+y] and [−y] pulses being inserted into the−x/ΔIP/x sequence. The sequence may further comprise a second additionalpair of pulses of the form [+z][−z], where z is an impulse valuedifferent from y and may be either negative or positive, the [+z] and[−z] pulses being inserted into the −x/ΔIP/x sequence. The −x/ΔIP/xsequences may further comprise a period when no voltage is applied tothe pixel. This “no voltage” period may occur between two elements ofthe −x/ΔIP/x sequence, or within a single element thereof. The −x/ΔIP/xsequences may include two or more “no voltage” periods.

When using the aforementioned −x/ΔIP/x sequences, the display maycomprise a plurality of pixels divided into a plurality of groups, andthe transition may be effected by (a) selecting each of the plurality ofgroups of pixels in succession and applying to each of the pixels in theselected group either a drive voltage or a non-drive voltage, thescanning of all the groups of pixels being completed in a first frameperiod; (b) repeating the scanning of the groups of pixels during asecond frame period; and (c) interrupting the scanning of the groups ofpixels during a pause period between the first and second frame periods,this pause period being not longer than the first or second frameperiod.

In the limited transitions method, the rewriting of the display may beeffected such that a transition to a given gray level is always effectedby a final pulse of the same polarity. In particular, gray levels otherthan the two extreme optical states may be approached from the directionof the nearer extreme optical state.

This invention also provides a method for driving an electro-opticdisplay having a plurality of pixels divided into a plurality of groups.This method comprises:

-   -   (a) selecting each of the plurality of groups of pixels in        succession and applying to each of the pixels in the selected        group either a drive voltage or a non-drive voltage, the        scanning of all the groups of pixels being completed in a first        frame period;    -   (b) repeating the scanning of the groups of pixels during a        second frame period; and    -   (c) interrupting the scanning of the groups of pixels during a        pause period between the first and second frame periods, this        pause period being not longer than the first or second frame        period.

This method may hereinafter for convenience be referred to as the“interrupted scanning” method of the present invention.

In such an interrupted scanning method, typically the first and secondframe periods are equal in length. The length of the pause period may bea sub-multiple of the length of one of the first and second frameperiods. The interrupted scanning method may include multiple pauseperiods; thus the method may comprise scanning the groups of pixelsduring at least first, second and third frame periods, and interruptingthe scanning of the groups of pixels during at least first and secondpause periods between successive frame periods. The first, second andthird frame periods may be substantially equal in length, and the totallength of the pause periods be equal to one frame period or one frameperiod minus one pause period. Typically, in the interrupted scanningmethod, the pixels are arranged in a matrix having a plurality of rowsand a plurality of columns with each pixel defined by the intersectionof a given row and a given column, and each group of pixels comprisesone row or one column of the matrix. The interrupted scanning method ispreferably DC balanced, i.e., the scanning of the display is preferablyeffected such that, for any series of transitions undergone by a pixel,the integral of the applied voltage with time is bounded.

In another aspect, this invention provides a method for driving anelectro-optic display having a plurality of pixels, the pixels beingdriven with a pulse width modulated waveform capable of applying aplurality of differing impulses to each pixel. This method comprises:

-   -   (a) storing data indicating whether application of a given        impulse to a pixel will produce a gray level higher or lower        than a desired gray level;    -   (b) detecting when two adjacent pixels are both required to be        in the same gray level; and    -   (c) adjusting the impulses applied to the two pixels so that one        pixel is below the desired gray level, while the other pixel is        above the desired gray level.

This method may hereinafter for convenience be referred to as the“balanced gray level” method of the present invention.

In this method, the pixels may be divided into two groups such that eachpixel has at least one neighbor of the opposite group, and differentdrive schemes be used for the two groups.

Each the methods of the present invention as described above may becarried out with any of the aforementioned types of electro-optic media.Thus, the methods of the present invention may be used withelectro-optic displays comprising an electrochromic or rotatingbichromal member electro-optic medium, an encapsulated electrophoreticmedium, or a microcell electrophoretic medium. Other types ofelectro-optic media may also be employed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an apparatus of the presentinvention, a display which is being driven by the apparatus, andassociated apparatus, and is designed to show the overall architectureof the system.

FIG. 2 is a schematic block diagram of the controller unit shown in FIG.1 and illustrates the output signals generated by this unit.

FIG. 3 is a schematic block diagram showing the manner in which thecontroller unit shown in FIGS. 1 and 2 generates certain output signalsshown in FIG. 2.

FIGS. 4 and 5 illustrate two different sets of reference voltages whichcan be used in the display shown in FIG. 1.

FIG. 6 is a schematic representation of tradeoffs between pulse widthmodulation and voltage modulation approaches in a look-up table methodof the present invention.

FIG. 7 is a block diagram of a custom driver useful in a look-up tablemethod of the present invention.

FIG. 8 is a flow chart illustrating a program which may be run by thecontroller unit shown in FIGS. 1 and 2.

FIGS. 9 and 10 illustrate two drive schemes of the present invention.

FIGS. 11A and 11B illustrate two parts of a further drive scheme of thepresent invention.

FIG. 12 illustrates the preferred −x/ΔIP/x sequence for use in themethods of the present invention.

FIG. 13 illustrates schematically how the waveform shown in FIG. 12 maybe modified to include an additional pair of drive pulses.

FIG. 14 illustrates one waveform produced by modifying the waveform ofFIG. 12 in the manner illustrated in FIG. 13.

FIG. 15 illustrates a second waveform produced by modifying the waveformof FIG. 12 in the manner illustrated in FIG. 13.

FIG. 16 illustrates schematically how the waveform shown in FIG. 15 maybe further modified to include an additional pair of drive pulses.

FIG. 17 illustrates one waveform produced by modifying the waveform ofFIG. 15 in the manner illustrated in FIG. 16.

FIGS. 18-20 illustrate three modifications of the waveform shown in FIG.12 to incorporate a period of zero voltage.

FIGS. 21A-21E show five non contiguous waveforms which can be used inthe methods of the present invention.

FIG. 22 illustrates a problem in addressing an electro-optic displayusing various numbers of frames of a monopolar voltage.

FIG. 23 illustrates one approach to solving the problem shown in FIG. 22using a non-contiguous variant of a method of the present invention.

FIG. 24 illustrates a second approach to solving the problem shown inFIG. 13 using a non-contiguous variant of a method of the presentinvention.

FIG. 25 illustrates a waveform which may be used in a non-contiguousvariant of a method of the present invention.

FIG. 26 illustrates a base waveform which can be modified to produce thewaveform shown in FIG. 25.

FIG. 27 illustrates a problem in addressing an electro-optic displayusing various numbers of frames of a monopolar voltage while maintainingDC balance.

FIG. 28 illustrates one approach to solving the problem shown in FIG. 18using a non-contiguous addressing method.

FIG. 29 illustrates a second approach to solving the problem shown inFIG. 18 using the non-contiguous addressing method.

FIG. 30 illustrates the gray levels obtained in a nominally four graylevel electro-optic display without using a non-contiguous addressingmethod, as described in the Example below.

FIG. 31 illustrates the gray levels obtained from the same display as inFIG. 30 using various non-contiguous addressing sequences.

FIG. 32 illustrates the gray levels obtained from the same display as inFIG. 30 using a modified non-contiguous drive scheme.

FIG. 33 illustrates a simple DC balanced waveform which may be used todrive an electro-optic display.

FIGS. 34 and 35 illustrate two modifications of the waveform shown inFIG. 33 to incorporate a period of zero voltage.

FIG. 36 illustrates schematically how the waveform shown in FIG. 33 maybe modified to include an additional pair of drive pulses.

FIG. 37 illustrates one waveform produced by modifying the waveform ofFIG. 33 in the manner illustrated in FIG. 36.

FIG. 38 illustrates a second waveform produced by modifying the waveformof FIG. 33 in the manner illustrated in FIG. 36.

FIG. 39 illustrates schematically how the waveform shown in FIG. 38 maybe further modified to include a third pair of drive pulses.

FIG. 40 illustrates one waveform produced by modifying the waveform ofFIG. 38 in the manner illustrated in FIG. 39.

FIG. 41 is a graph illustrating the reduced dwell time dependency whichcan be achieved by a compensation voltage method.

FIG. 42 is a graph illustrating the effect of dwell time dependence inan electro-optic display.

FIGS. 43A and 43B illustrate respectively transitions occurring during aprior art drive scheme and a limited transitions method of the presentinvention.

DETAILED DESCRIPTION

From the foregoing, it will be apparent that the present inventionprovides several different improvements in methods for drivingelectro-optic displays. In the description below, the various differentimprovements provided by the present invention will normally bedescribed separately, although it will be understood by those skilled inthe imaging art that in practice a single display may make use of morethan one of these major aspects; for example, a display which uses thelimited transitions method of the present invention may also make use ofthe interrupted scanning method. Furthermore, since the improvementsprovided by the present invention can be applied to a wide variety ofmethods for driving electro-optic displays described in the applicationsmentioned in Paragraphs [0001] to [0004] hereof, including such featuresas temperature compensation and the like, it is deemed desirable, beforesetting out the details of the present improved methods, to given ageneral introduction describing these prior art methods.

General Introduction

As already mentioned, the methods of the present invention relate todriving electro-optic displays, typically having a plurality of pixels,each of which is capable of displaying at least three gray levels. Thepresent methods may of course be applied to electro-optic displayshaving a greater number of gray levels, for example 4, 8, 16 or more.

Also as already mentioned, driving bistable electro-optic displaysrequires very different methods from those normally used to drive liquidcrystal displays (“LCD's”). In a conventional (non-cholesteric) LCD,applying a specific voltage to a pixel for a sufficient period willcause the pixel to attain a specific gray level. Furthermore, the liquidmaterial is only sensitive to the magnitude of the electric field, notits polarity. In contrast, bistable electro-optic displays act asimpulse transducers, so there is no one-to-one mapping between appliedvoltage and gray state attained; the impulse (and thus the voltage)which must be applied to a pixel to achieve a given gray state varieswith the “initial” gray state of the relevant pixel. Furthermore, sincebistable electro-optic displays need to be driven in both directions(white to black, and black to white) it is necessary to specify both thepolarity and the magnitude of the impulse needed.

At this point, it is considered desirable to define certain terms whichare used herein in accordance with their conventional meaning in thedisplay art. Most of the discussion below will concentrate upon one ormore pixels of a display undergoing a single gray scale transition(i.e., a change from one gray level to another) from an “initial” stateto a “final” state. Obviously, the initial state and the final state areso designated only with regard to the particular single transition beingconsidered and in most cases the pixel with have undergone transitionsprior to the “initial” state and will undergo further transitions afterthe “final” state. As explained below, some methods of the inventiontake account not only of the initial and final states of the pixel butalso of “prior” states, in which the pixel existed prior to achievingthe initial state. Where it is necessary to distinguish between multipleprior states, the term “first prior state” will be used to refer to thestate in which the relevant pixel existed prior to the initial state,the term “second prior state” will be used to refer to the state inwhich the relevant pixel existed prior to the first prior state, and soon. The term “non-zero transition” is used to refer to a transitionwhich effects a change of at least one unit in gray scale; the term“zero transition” may be used to refer to a “transition” which effectsno overall change in gray scale of the selected pixel (although the graylevel of the pixel may vary during the transition, the final gray levelof the pixel after the transition is the same as the initial gray levelthereof prior to the transition; also, of course, other pixels of thedisplay may be undergoing non-zero transitions at the same time). Asdiscussed in more detail below, prior states which may be taken intoaccount in the methods of the present invention are of two types, namely“gray level” prior states (i.e., states determined a specific number ofnon-zero transitions prior to the transition being considered) and“temporal” prior states (i.e., states determined a specific time priorto the transition being considered).

As will readily be apparent to those skilled in image processing, amethod of the present invention may take account of only of the initialstate of each pixel and the final state, and such a method may make useof a look-up table, which will be two-dimensional. However, as alreadymentioned, some electro-optic media display a memory effect and withsuch media it is desirable, when generating the output signalrepresentative of the pulse or series of pulses to be applied to a pixelto effect a transition, to take into account not only the initial stateof each pixel but also at least one prior state of the same pixel, inwhich case the look-up table will be three-dimensional. In some cases,it may be desirable to take into account more than one prior state ofeach pixel (the plurality of prior states thus taken into account may beany combination of gray level and temporal prior states), thus resultingin a look-up table having four (if only two prior states are taken intoaccount) or more dimensions.

From a formal mathematical point of view, the present methods may beregarded as using an algorithm that, given information about theinitial, final and (optionally) prior states of an electro-optic pixel,as well as (optionally—see more detailed discussion below) informationabout the physical state of the display (e.g., temperature and totaloperating time), will produce a function V(t) which can be applied tothe pixel to effect a transition to the desired final state. From thisformal point of view, a device controller used to carry out the presentmethods may be regarded as essentially a physical embodiment of thisalgorithm, the controller serving as an interface between a devicewishing to display information and an electro-optic display.

Ignoring the physical state information for the moment, the algorithmis, in accordance with preferred methods of the present invention,encoded in the form of a look-up table or transition matrix. This matrixwill have one dimension each for the desired final state, and for eachof the other states (initial and any prior states) are used in thecalculation. The elements of the matrix will contain a function V(t)that is to be applied to the electro-optic medium.

The elements of the look-up table or transition matrix may have avariety of forms. In some cases, each element may comprise a singlenumber. For example, an electro-optic display may use a high precisionvoltage modulated driver circuit capable of outputting numerousdifferent voltages both above and below a reference voltage, and simplyapply the required voltage to a pixel for a standard, predeterminedperiod. In such a case, each entry in the look-up table could simplyhave the form of a signed integer specifying which voltage is to beapplied to a given pixel. In other cases, each element may comprise aseries of numbers relating to different portions of a waveform. Forexample, there are described below embodiments of the invention whichuse single- or double-prepulse waveforms, and specifying such a waveformnecessarily requires several numbers relating to different portions ofthe waveform. Also described below is an embodiment of the inventionwhich in effect applies pulse length modulation by applying apredetermined voltage to a pixel during selected ones of a plurality ofsub-scan periods (frames) during a complete scan (superframe). In suchan embodiment, the elements of the transition matrix may have the formof a series of bits specifying whether or not the predetermined voltageis to be applied during each sub-scan period (frame) of the relevanttransition. Finally, as discussed in more detail below, in some cases,such as a temperature-compensated display, it may be convenient for theelements of the look-up table to be in the form of functions (or, inpractice, more accurately coefficients of various terms in suchfunctions).

It will be apparent that the look-up tables used in some embodiments ofthe invention may become very large. To take an extreme example,consider a process of the invention for a 256 (28) gray level displayusing an algorithm that takes account of initial, final and two priorstates. The necessary four-dimensional look-up table has 232 entries. Ifeach entry requires (say) 64 bits (8 bytes), the total size of thelook-up table would be approximately 32 Gbyte. While storing this amountof data poses no problems on a desktop computer, it may present problemsin a portable device. However, in practice the size of such largelook-up tables can be substantially reduced. In many instances, it hasbeen found that there are only a small number of types of waveformsneeded for a large number of different transitions, with, for example,the length of individual pulses of a general waveform being variedbetween different transitions. Consequently, the length of individualentries in the look-up table can be reduced by making each entrycomprises (a) a pointer to an entry in a second table specifying one ofa small number of types of waveform to be used; and (b) a small numberof parameters specifying how this general waveform should be varied forthe relevant transition.

The values for the entries in the look-up table may be determined inadvance through an empirical optimization process. Essentially, one setsa pixel to the relevant initial state, applies an impulse estimated toapproximately equal that needed to achieve the desired final state andmeasures the final state of the pixel to determine the deviation, ifany, between the actual and desired final state. The process is thenrepeated with a modified impulse until the deviation is less than apredetermined value, which may be determined by the capability of theinstrument used to measure the final state. In the case of methods whichtake into account one or more prior states of the pixel, in addition tothe initial state, it will generally be convenient to first determinethe impulse needed for a particular transition when the state of thepixel is constant in the initial state and all preceding states used indetermining the impulse, and then to “fine tune” this impulse to allowfor differing previous states.

The methods of the present invention desirably provide for modificationof the impulse to allow for variation in temperature and/or totaloperating time of the display; compensation for operating time may berequired because some electro-optic media “age” and their behaviorchanges after extended operation. Such modification may be done in oneof two ways. Firstly, the look-up table may be expanded by an additionaldimension for each variable that is to be taken into account incalculating the output signal. Obviously, when dealing with continuousvariables such as temperature and operating time, it is necessary toquantize the continuous variable in order to maintain the look-up tableat a practicable finite size. In order to find the waveform to beapplied to the pixel, the calculation means may simply choose thelook-up table entry for the table closest to the measured temperature.Alternatively, to provide more accurate temperature compensation, thecalculation means may look up the two adjacent look-up table entries oneither side of the measured continuous variable, and apply anappropriate interpolation algorithm to calculate the required entry atthe measured intermediate value of the variable. For example, assumethat the matrix includes entries for temperature in increments of 10° C.If the actual temperature of the display is 25° C., the calculationwould look up the entries for 20° and 30° C., and use a valueintermediate the two. Note that since the variation of characteristicsof electro-optic media with temperature is often not linear, the set oftemperatures for which the look-up table stores entries may not bedistributed linearly; for example, the variation of many electro-opticmedia with temperature is most rapid at high temperatures, so that atlow temperatures intervals of 20° C. between look-up tables mightsuffice, whereas at high temperatures intervals of 5° C. might bedesirable.

An alternative method for temperature/operating time compensation is touse look-up table entries in the form of functions of the physicalvariable(s), or perhaps more accurately coefficients of standard termsin such functions. For simplicity consider the case of a display whichuses a time modulation drive scheme in which each transition is handledby applying a constant voltage (of either polarity) to each pixel for avariable length of time, so that, absent any correction forenvironmental variables, each entry in the look-up table could consistonly of a single signed number representing the duration of time forwhich the constant voltage is to be applied, and its polarity. If it isdesired to correct such a display for variations in temperature suchthat the time T_(t) for which the constant voltage needs to be appliedfor a specific transition at a temperature t is given by:T _(t) =T ₀ +AΔt+B(Δt)²where T₀ is the time required at some standard temperature, typicallythe mid-point of the intended operating temperature range of thedisplay, and Δt is the difference between t and the temperature at whichT₀ is measured; the entries in the look-up table can consist of thevalues of T₀, A and B for the specific transition to which a given entryrelates, and the calculation means can use these coefficients tocalculate T_(t) at the measured temperature. To put it more generally,the calculation means finds the appropriate look-up table entry for therelevant initial and final states, then uses the function defined bythat entry to calculate the proper output signal having regard to theother variables to be taken into account.

The relevant temperature to be used for temperature compensationcalculations is that of the electro-optic material at the relevantpixel, and this temperature may differ significantly from ambienttemperature, especially in the case of displays intended for outdoor usewhere, for example, sunlight acting through a protective front sheet maycause the temperature of the electro-optic layer to be substantiallyhigher than ambient. Indeed, in the case of large billboard-type outdoorsigns, the temperature may vary between different pixels of the samedisplay if, for example, part of the display falls within the shadow ofan adjacent building, while the remainder is in full sunlight.Accordingly, it may be desirable to embed one or more thermocouples orother temperature sensors within or adjacent to the electro-optic layerto determine the actual temperature of this layer. In the case of largedisplays, it may also be desirable to provide for interpolation betweentemperatures sensed by a plurality of temperature sensors to estimatethe temperature of each particular pixel. Finally, in the case of largedisplays formed from a plurality of modules which can replacedindividually, the method and controller of the invention may provide fordifferent operating times for pixels in different modules.

The methods of the present invention may also allow for the residencetime (i.e., the period since the pixel last underwent a non-zerotransition) of the specific pixel being driven. It has been found that,at least in some cases, the impulse necessary for a given transitionvarious with the residence time of a pixel in its optical state, thisphenomenon, which does not appear to have previously been discussed inthe literature, hereinafter being referred to as “dwell time dependence”or “DTD”, although the term “dwell time sensitivity” was used in theaforementioned Application Ser. No. 60/320,070. Thus, it may bedesirable or even in some cases in practice necessary to vary theimpulse applied for a given transition as a function of the residencetime of the pixel in its initial optical state. In one approach toallowing for DTD, the look-up table contains an additional dimension,which is indexed by a counter indicating the residence time of the pixelin its initial optical state. In addition, the controller may require anadditional storage area that contains a counter for every pixel in thedisplay, and a display clock, which increments by one the counter valuestored in each pixel at a set interval. The length of this interval mustbe an integral multiple of the frame time of the display, and thereforemust be no less than one frame time. (The frame time of the display maynot be constant, but instead may vary from scan to scan, by adjustingeither the line time or the delay period at the end of the frame. Inthis case, the relationship between the frame counter and the elapsedtime may be calculated by summing the frame times for the individualframes comprising the update.) The size of this counter and the clockfrequency will be determined by the length of time over which theapplied impulse will be varied, and the necessary time resolution. Forexample, storing a 4-bit counter for each pixel would allow the impulseto vary at 0.25 second intervals over a 4-second period (4 seconds*4counts/sec=16 counts=4 bits). The counter may optionally be reset uponthe occurrence of certain events, such as the transition of the pixel toa new state. Upon reaching its maximum value, the counter may beconfigured to either “roll over” to a count of zero, or to maintain itsmaximum value until it is reset.

The methods of the present invention may take account of not only theinitial state of the relevant pixel and one or more gray level priorstates of the same pixel, but also one or more temporal prior states ofthe pixel, i.e., data representing the state of the relevant pixel atdefined points in time prior to the transition being considered. Theoutput signal from the method is determined dependent upon the graylevel and temporal prior states, and the initial state of the pixel.

Allowing for both the gray state levels in which a given pixel existedprior to the initial state and the length of time for which the pixelremained in those gray levels reduces “image drift” (i.e., inaccuracy ingray levels). It is believed (although the invention is in no waylimited by this belief) that such image drift is due to polarizationwithin the electro-optic medium.

Table 1 below illustrates a relatively simple application of a priortemporal/gray level state method to a two-bit (four gray level) grayscale display in which the various gray levels of denoted 0 (black), 1(dark gray), 2 (light gray) and 3 (white). (Obviously, the method can beapplied to applied to displays having large numbers of gray levels, forexample a four-bit, 16 gray level, display having gray levels denotedfrom 0 (black) to 15 (white).) The middle line of Table 1 showssuccessive gray levels of a single pixel of the display; Table 1 assumesthat the display is being updated continuously, so that the intervalbetween adjacent columns of the display is one superframe (i.e., theinterval necessary for a complete updating of the display). Obviously,if the present invention is applied to a display of a type (for example,a weather radar display) in which each updating if followed by a restinterval during which no rewriting of the display is effected, theinterval between columns of Table 1 would be to be taken as onesuperframe plus the associated rest interval.

TABLE 1 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 2 0 0 0 3 3 1 1 1 2 R5 R4 R4 R4R3 R3 R2 R2 R2 R1

The top line of Table 1 shows the various temporal states Sx of thedisplay, while the bottom of the table shows the corresponding graylevel states Rx, the difference being that the temporal states change atintervals of one superframe, whereas the gray level states change onlywhen there is a change in gray level (non-zero transition) of therelevant pixel. The right hand column of Table 1 represents the desiredfinal state of the display after the transition being considered, whilethe penultimate column represents the initial state prior to thistransition. Table 1 assumes a non-zero transition (i.e., that the finalgray level is different from the initial gray level), since, at least insome cases, a zero transition in any one pixel of a bistableelectro-optic display may be effected simply by not applying any voltageto the pixel during the relevant superframe.

Thus,

S1=R1=the desired final state of the pixel;

S2=R2=the initial state of the pixel;

S3=the first temporal prior state of the pixel;

S4=the second temporal prior state of the pixel; and similarly for S5 toS10, while:

R3=the first gray level prior state of the pixel;

R4=the second gray level prior state of the pixel; and

R5=the third gray level prior state of the pixel.

The basic look-up table method described in the aforementioned2003/0137521 uses a look-up table indexed by (i.e., having dimensionscorresponding to) R1 and R2, and optionally any one or more successiveones of R3, R4 and R5. In contrast, the prior temporal/gray level statemethod uses a look-up table indexed by at least R1 (=S1), R2 (=S2), R3and S3. Optionally, the prior temporal/gray level state method may use alook-up table indexed by any one or more successive ones of R4, R5 etc.,and any one or more successive ones of S4, S5 etc. It is not necessarythat the prior temporal/gray level state method take account of an equalnumber of temporal and gray level prior states, nor is it necessary thatthe prior temporal/gray level state method take account of successivetemporal prior states extending over the same time interval as the graylevel prior states of which the method takes account. Indeed, since thevariations in impulse due to changes in temporal prior states tend to besmaller than those due to changes in gray level prior states, it may,for example, in some cases be advantageous for the prior temporal/graylevel state method to take account of (say) the first and second graylevel prior states (R3 and R4 respectively) and only the first temporalprior state (S3), even though clearly the second gray level prior stateR4 occurs at a time prior to the first temporal prior state S3.

As compared with the basic look-up table method, the prior temporal/graylevel state method allows better compensation for effects (such aspolarization fields building up with the electro-optic medium) due tothe electro-optic medium “dwelling” in particular gray states forextended periods. This better compensation can reduce the overallcomplexity of the display controller and/or reduce the magnitude ofimage artifacts such as prior state ghosting.

The prior temporal/gray level state method may make use of any of theoptional features of the basic look-up table method described above.Thus, the elements of the look-up table or transition matrix may have avariety of forms. In some cases, each element may comprise a singlenumber. In other cases, each element may comprise a series of numbersrelating to different portions of a waveform. In still other cases, suchas a temperature-compensated display, it may be convenient for theelements of the look-up table to be in the form of functions (or, inpractice, more accurately coefficients of various terms in suchfunctions). Similarly, to prevent the look-up tables becoming too large,the length of individual entries in the look-up table may be reduced bymaking each entry (a) a pointer to an entry in a second table specifyingone of a small number of types of waveform to be used; and (b) a smallnumber of parameters specifying how this general waveform should bevaried for the relevant transition. Furthermore, since the datacomprising a look-up table can be treated as a general multi-dimensionaldata set, any standard functions, algorithms and encodings known tothose skilled in the art of data storage and processing may be employedto reduce one or more of (a) the size of the storage required for thedata set, (b) the computational effort required to extract the data, or(c) the time required to locate and extract a specific element from theset. These storage techniques include, for example, hash functions,loss-less and lossy compression, and representation of the data set as acombination of basic functions.

The values for the entries in the look-up table used in the priortemporal/gray level state method may be determined in advance through anempirical optimization process essentially similar to that describedabove for the basic look-up table method, although of course modified toallow for consideration of the one or more temporal prior statesconsidered. To take into account the required number of temporal andgray level prior states of the pixel, it will generally be convenient tofirst determine the impulse needed for a particular transition when thestate of the pixel is constant in the initial state and all prior statesused in determining the impulse, and then to “fine tune” this impulse toallow for differing temporal and gray level prior states.

The prior temporal/gray level state method desirably provides formodification of the impulse to allow for variation in temperature and/ortotal operating time of the display, in exactly the same way asdescribed above for the basic look-up table method. Prior state,temperature, operation time and other external variables may be used tomodify the structure of the transitions comprising the waveform, forexample by inserting 0 V periods within a transition, while leaving thenet impulse unchanged.

Both the basic look-up table method and the prior temporal/gray levelstate method may of course be modified to take account of any otherphysical parameter which has a detectable effect upon the impulse neededto effect any one or more specific transitions of an electro-opticmedium. For example, the method could be modified to incorporatecorrections for ambient humidity if the electro-optic medium is found tobe sensitive to humidity.

For a bistable electro-optic medium, the look-up table may have thecharacteristic that, for any zero transition in which the initial andfinal states of the pixel are the same, the entry will be zero, or inother words, no voltage will be applied to the pixel. As a corollary, ifno pixels on the display change during a given interval, then noimpulses need be applied. This enables ultra-low power operation, aswell as ensuring that the electro-optic medium is not overdriven while astatic image is being displayed. In general, the look-up table may onlyretain information about non-null transitions. In other words, for twoimages, I and I+1, if a given pixel is in the same state in I and I+1,then state I+1 need not be stored in the prior state table, and nofurther information need be stored until that pixel undergoes atransition. However, as discussed below, at least in some cases it maystill be advantageous to apply impulses to pixels undergoing zerotransitions.

The look-up table methods described above can be practiced withcontrollers having a variety of physical forms, and using anyconventional data processing components. For example, the methods couldbe practiced using a general purpose digital computer in conjunctionwith appropriate equipment (for example, one or more digital analogconverters, “DAC's”) to convert the digital outputs from the computer toappropriate voltages for application to pixels. Alternatively, themethods could be practiced using an application specific integratedcircuit (ASIC). In particular, the controller could have the form of avideo card which could be inserted into a personal computer to enablethe images generated by the computer to be displayed on an electro-opticscreen instead of or in addition to an existing screen, such as a LCD.Since the construction of the controller is well within the level ofskill in the image processing art, it is unnecessary to describe itscircuitry in detail herein.

A preferred physical embodiment of the controller is a timing controllerintegrated circuit (IC). This IC accepts incoming image data and outputscontrol signals to a collection of data and select driver IC's, in orderto produce the proper voltages at the pixels to produce the desiredimage. This IC may accept the image data through access to a memorybuffer that contains the image data, or it may receive a signal intendedto drive a traditional LCD panel, from which it can extract the imagedata. It may also receive any serial signal containing information thatit requires to perform the necessary impulse calculations. Alternately,this timing controller can be implemented in software, or incorporatedas a part of the CPU. The timing controller may also have the ability tomeasure any external parameters that influence the operation of thedisplay, such as temperature.

The controller can operate as follows. The look-up table(s) are storedin memory accessible to the controller. For each pixel in turn, all ofthe necessary initial, final and (optionally) prior and physical stateinformation is supplied as inputs. The state information is then used tocompute an index into the look-up table. In the case of quantizedtemperature or other correction, the return value from a look-up usingthis index will be one voltage, or an array of voltages versus time. Thecontroller will repeat this process for the two bracketing temperaturesin the look-up table, then interpolate between the values. For thealgorithmic temperature correction, the return value of the look-up willbe one or more parameters, which can then be inserted into an equationalong with the temperature, to determine the proper form of the driveimpulse, as already described. This procedure can be accomplishedsimilarly for any other system variables that require real-timemodification of the drive impulse. One or more of these system variablesmay be determined by, for example, the value of a programmable resistor,or a memory location in an EPROM, which is set on the display panel atthe time of construction in order to optimize the performance of thedisplay.

An important feature of the display controller is that, unlike mostdisplays, in most practical cases several complete scans of the displaywill be required in order to complete an image update. The series ofscans required for one image update should be considered to be anuninterruptible unit. If the display controller and image source areoperating asynchronously, then the controller must ensure that the databeing used to calculate applied impulses remains constant across allscans. This can be accomplished in one of two ways. Firstly, theincoming image data could be stored in a separate buffer by the displaycontroller (alternatively, if the display controller is accessing adisplay buffer through dual-ported memory, it could lock out access fromthe CPU). Secondly, on the first scan, the controller may store thecalculated impulses in an impulse buffer. The second option has theadvantage that the overhead for scanning the panel is only incurred onceper transition, and the data for the remaining scans can be outputdirectly from the buffer.

Optionally, imaging updating may be conducted in an asynchronous manner.Although it will, in general, take several scans to effect a completetransition between two images, individual pixels can begin transitions,or reverse transitions that have already started, in mid-superframe. Inorder to accomplish this, the controller must keep track of what portionof the total transition have been accomplished for a given pixel. If arequest is received to change the optical state of a pixel that is notcurrently in transition, then the counter for that pixel can be set tozero, and the pixel will begin transitioning on the next frame. If thepixel is actively transitioning when a new request is received, then thecontroller will apply an algorithm to determine how to reach the newstate from the current mid-transition state. This may be effected, forexample, by adding an extra dimension to the look-up table to indicatehow many frames into the update a given pixel is before the request totransition to a new state is given. In this way, transitions can bespecified not just between final gray states, but also betweenintermediate points in any transition to a new final gray state.

In order to minimize the power necessary to operate a display, and tomaximize the image stability of the electro-optic medium, the displaycontroller may stop scanning the display and reduce the voltage appliedto all pixels to, or close to, zero, when there are no pixels in thedisplay that are undergoing transitions. Very advantageously, thedisplay controller may turn off the power to its associated row andcolumn drivers while the display is in such a “hold” state, thusminimizing power consumption. In this scheme, the drivers would bereactivated when the next pixel transition is requested.

FIG. 1 of the accompanying drawings shows schematically an apparatus,useful for carrying out the driving methods of the present invention, inuse, together with associated apparatus. The overall apparatus(generally designated 10) shown in FIG. 1 comprises an image source,shown as a personal computer 12 which outputs on a data line 14 datarepresenting an image. The data line 14 can be of any conventional typeand may be a single data line or a bus; for example, the data line 14could comprise a universal serial bus (USB), serial, parallel, IEEE-1394or other line. The data which are placed on the line 14 can be in theform of a conventional bit mapped image, for example a bit map (BMP),tagged image file format (TIF), graphics interchange format (GIF) orJoint Photographic Experts Group (JPEG) file. Alternatively, however,the data placed on the line 14 could be in the form of signals intendedfor driving a video device; for example, many computers provide a videooutput for driving an external monitor and signals on such outputs maybe used in the present invention. It will be apparent to those skilledin imaging processing that the apparatus described below may have toperform substantial file format conversion and/or decoding to make useof the disparate types of input signals which can be used, but suchconversion and/or decoding is well within the level of skill in the art,and accordingly, the apparatus will be described only from the point atwhich the image data used as its original inputs have been converted toa format in which they can be processed by the apparatus.

The data line 14 extends to a controller unit 16, as described in detailbelow. This controller unit 16 generates one set of output signals on adata bus 18 and a second set of signals on a separate data bus 20. Thedata bus 18 is connected to two row (or gate) drivers 22, while the databus 20 is connected to a plurality of column (or source) drivers 24.(The number of row drivers 22 and column drivers 24 is greatly reducedin FIG. 1 for ease of illustration.) The row and column drivers controlthe operation of a bistable electro-optic display 26.

The apparatus shown in FIG. 1 is chosen to illustrate the various unitsused, and is most suitable for a developmental, “breadboard” unit. Inactual commercial production, the controller 16 will typically be partof the same physical unit as the display 26, and the image source mayalso be part of this physical unit, as in conventional laptop computersequipped with LCD's, and in personal digital assistants. Also, theapparatus is illustrated in FIG. 1, and will be mainly described below,in conjunction with an active matrix display architecture which has asingle common, transparent electrode (not shown in FIG. 1) on one sideof the electro-optic layer, this common electrode extending across allthe pixels of the display. Typically, this common electrode lies betweenthe electro-optic layer and the observer and forms a viewing surfacethrough which an observer views the display. On the opposed side of theelectro-optic layer is disposed a matrix of pixel electrodes arranged inrows and columns such that each pixel electrode is uniquely defined bythe intersection of a single row and a single column. Thus, the electricfield experienced by each pixel of the electro-optic layer is controlledby varying the voltage applied to the associated pixel electroderelative to the voltage (normally designated “Vcom”) applied to thecommon front electrode. Each pixel electrode is associated with at leastone transistor, typically a thin film transistor. The gates of thetransistors in each row are connected via a single elongate rowelectrode to one of the row drivers 22. The source electrodes of thetransistors in each column are connected via a single elongate columnelectrode to one of column drivers 24. The drain electrode of eachtransistor is connected directly to the pixel electrode. It will beappreciated that the assignment of the gates to rows and the sourceelectrodes to columns is arbitrary, and could be reversed, as could theassignment of source and drain electrodes. However, the followingdescription will assume the conventional assignments.

During operation, the row drivers 22 apply voltages to the gates suchthat the transistors in one and only one row are conductive at any giventime. Simultaneously, the column drivers 24 apply predetermined voltagesto each of the column electrodes. Thus, the voltages applied to thecolumn drivers are applied to only one row of the pixel electrodes, thuswriting (or at least partially writing) one line of the desired image onthe electro-optic medium. The row driver then shifts to make thetransistors in the next row conductive, a different set of voltages areapplied to the column electrodes, and the next line of the image iswritten.

It is emphasized that the methods of the present invention are notconfined to such active matrix displays. Once the correct waveforms foreach pixel of the image have been determined in accordance with themethods of the present invention, any switching scheme may be used toapply the waveforms to the pixels. For example, the present methods canbe used in a so-called “direct drive” scheme, in which each pixel isprovided with a separate drive line. In principle, the present methodscan also be used in a passive matrix drive scheme of the type used insome LCD's, but it should be noted that, since many bistableelectro-optic media lack a threshold for switching (i.e., the media willchange optical state if even a small electric field is applied for aprolonged period), such media are unsuitable for passive matrix driving.However, since it appears that the present methods will find their majorapplication in active matrix displays, they will be described hereinprimarily with reference to such displays.

The controller unit 16 (FIG. 1) has two main functions. Firstly, usingthe methods of the present invention, the controller calculates atwo-dimensional matrix of impulses (or waveforms) which must be appliedto the pixels of a display to change an initial image to a final image.Secondly, the controller 16 calculates, from this matrix of impulses,all the timing signals necessary to provide the desired impulses at thepixel electrodes to drive a bistable electro-optic display.

As shown in FIG. 2, the controller unit 16 seen in FIG. 1 has two mainsections, namely a frame buffer 16A, which buffers the data representingthe final image which the controller 16B is to write to the display 26(FIG. 1), and the controller proper, denoted 16B. The controller 16Breads data from the buffer 16A pixel by pixel and generates varioussignals on the data buses 18 and 20 as described below.

The signals shown in FIG. 2 are as follows:

D0:D5—a six-bit voltage value for a pixel (obviously, the number of bitsin this signal may vary depending upon the specific row and columndrivers used)

POL—pixel polarity with respect to Vcom (see below)

START—places a start bit into the column driver 24 to enable loading ofpixel values

HSYNC—horizontal synchronization signal, which latches the column driver

PCLK—pixel clock, which shifts the start bit along the row driver

VSYNC—vertical synchronization signal, which loads a start bit into therow driver

OE—output enable signal, which latches the row driver.

Of these signals, VSYNC and OE supplied to the row drivers 22 areessentially the same as the corresponding signals supplied to the rowdrivers in a conventional active matrix LCD, since the manner ofscanning the rows in the apparatus shown in FIG. 1 is in principleidentical to the manner of scanning an LCD, although of course the exacttiming of these signals may vary depending upon the preciseelectro-optic medium used. Similarly, the START, HSYNC and PCLK signalssupplied to the column drivers are essentially the same as thecorresponding signals supplied to the column drivers in a conventionalactive matrix LCD, although their exact timing may vary depending uponthe precise electro-optic medium used. Hence, it is considered that nofurther description of these output signals in necessary.

FIG. 3 illustrates, in a highly schematic manner, the way in which thecontroller 16B shown in FIG. 2 generates the D0:D5 and POL signals. Asdescribed above, the controller 16B stores data representing the finalimage 120 (the image which it is desired to write to the display), theinitial image 122 previously written to the display, and optionally oneor more prior images 123 which were written to the display before theinitial image. The embodiment of the invention shown in FIG. 3 storestwo such prior images 123. (Obviously, the necessary data storage can bewithin the controller 16B or in an external data storage device.) Thecontroller 16B uses the data for a specific pixel (illustrated as thefirst pixel in the first row, as shown by the shading in FIG. 3) in theinitial, final and prior images 120, 122 and 123 as pointers into alook-up table 124, which provides the value of the impulse which must beapplied to the specific pixel to change the state of that pixel to thedesired gray level in the final image. The resultant output from thelook-up table 124, and the output from a frame counter 126, are suppliedto a voltage v. frame array 128, which generates the D0:D5 and POLsignals.

The controller 16B (FIG. 2) is designed for use with a TFT LCD driverthat is equipped with pixel inversion circuitry, which ordinarilyalternates the polarity of neighboring pixels with respect to the topplane. Alternate pixels will be designated as even and odd, and areconnected to opposing sides of the voltage ladder. Furthermore, a driverinput, labeled “polarity”, serves to switch the polarity of the even andodd pixels. The driver is provided with four or more gamma voltagelevels, which can be set to determine the local slope of thevoltage-level curve. A representative example of a commercial integratedcircuit (IC) with these features is the Samsung KS0652 300/309 channelTFT-LCD source driver. As previously discussed, the display to be drivenuses a common electrode on one side of the electro-optic medium, thevoltage applied to this common electrode being referred to as the “topplane voltage” or “Vcom”.

In one embodiment, illustrated in FIG. 4 of the accompanying drawings,the reference voltages of the driver are arranged so that the top planevoltage is placed at one half the maximum voltage (Vmax) which thedriver can supply, i.e.,Vcom=Vmax/2and the gamma voltages are arranged to vary linearly above and below thetop plane voltage. (FIGS. 4 and 5 are drawn assuming an odd number ofgamma voltages so that, for example, in FIG. 4 the gamma voltageVGMA(n/2+½) is equal to Vcom. If an even number of gamma voltages arepresent, both VGMA(n/2) and VGMA(n/2+1) are set equal to Vcom.Similarly, in FIG. 5, if an even number of gamma voltages are present,both VGMA(n/2) and VGMA(n/2+1) are set equal to the ground voltage Vss.)The pulse length necessary to achieve all needed transitions isdetermined by dividing the largest impulse needed to create the newimage by Vmax/2. This impulse can be converted into a number of framesby multiplying by the scan rate of the display. The necessary number offrames is then multiplied by two, to give an equal number of even andodd frames. These even and odd frames will correspond to whether thepolarity bit is set high or low for the frame. For each pixel in eachframe, the controller 16B must apply an algorithm which takes as itsinputs (1) whether the pixel is even or odd; (2) whether the polaritybit is high or low for the frame being considered; (3) whether thedesired impulse is positive or negative; and (4) the magnitude of thedesired impulse. The algorithm then determines whether the pixel can beaddressed with the desired polarity during that frame. If so, the properdrive voltage (impulse/pulse length) is applied to the pixel. If not,then the pixel is brought to the top plane voltage (Vmax/2) to place itin a hold state, in which no electric field is applied to the pixelduring that frame.

For example, consider two neighboring pixels in the display, an oddpixel 1 and an even pixel 2. Further, assume that when the polarity bitis high, the odd pixels will be able to access the positive drivevoltage range (i.e. above the top plane voltage), and the even pixelswill be able to access the negative voltages (i.e. below the top planevoltage). If both pixels 1 and 2 need to be driven with a positiveimpulse, then the following sequence must occur:

(a) during the positive polarity frames, pixel 1 is driven with apositive voltage, and pixel 2 is held at the top plane voltage; and

(b) during the negative polarity frames, pixel 1 is held at the topplane voltage, while pixel 2 is driven with a positive voltage.

Although typically frames with positive and negative polarity will beinterleaved 1:1 (i.e., will alternate with each other), but this is notnecessary; for example, all the odd frames could be grouped together,followed by all the even frames. This would result in alternate columnsof the display being driven in two separate groups.

The major advantage of this embodiment is that the common frontelectrode does not have to be switched during operation. The primarydisadvantage is that the maximum drive voltage available to theelectro-optic medium is only half of the maximum voltage of the driver,and that each line may only be driven 50% of the time. Thus, the refreshtime of such a display is four times the switching time of theelectro-optic medium under the same maximum drive voltage.

In a second embodiment of this form of the invention, the gamma voltagesof the driver are arranged as shown in FIG. 5, and the common electrodeswitches between V=0 and V=Vmax. Arranging the gamma voltages in thisway allows both even and odd pixels to be driven simultaneously in asingle direction, but requires that the common electrode be switched toaccess the opposite drive polarity. In addition, because thisarrangement is symmetric about the top plane voltage, a particular inputto the drivers will result in the same voltage being applied on eitheran odd or an even pixel. In this case, the inputs to the algorithm arethe magnitude and sign of the desired impulse, and the polarity of thetop plane. If the current common electrode setting corresponds to thesign of the desired impulse, then this value is output. If the desiredimpulse is in the opposite direction, then the pixel is set to the topplane voltage so that no electric field is applied to the pixel duringthat frame.

As in the embodiment previously described, in this embodiment thenecessary length of the drive pulse can be calculated by dividing themaximum impulse by the maximum drive voltage, and this value convertedinto frames by multiplying by the display refresh rate. Again, thenumber of frames must be doubled, to account for the fact that thedisplay can only be driven in one direction with respect to the topplane at a time.

The major advantage of this second embodiment is that the full voltageof the driver can be used, and all of the outputs can be driven at once.However, two frames are required for driving in opposed directions.Thus, the refresh time of such a display is twice the switching time ofthe electro-optic medium under the same maximum drive voltage. The majordrawback is the need to switch the common electrode, which may result inunwanted voltage artifacts in the electro-optic medium, the transistorsassociated with the pixel electrodes, or both.

In either embodiment, the gamma voltages are normally arranged on alinear ramp between the maximum voltages of the driver and the top planevoltage. Depending upon the design of the driver, it may be necessary toset one or more of the gamma voltages at the top plane value, in orderto ensure that the driver can actually produce the top plane voltage onthe output.

Reference has already been made above to the need to adapt the method ofthe present invention to the limitations of conventional driversdesigned for use with LCD's. More specifically, conventional columndrivers for LCD's, and particularly super twisted nematic (STN) LCD's(which can usually handle higher voltages than other types of columndrivers), are only capable of applying one of two voltages to a driveline at any given time, since this is all that a polarity-insensitive LCmaterial requires. In contrast, to drive polarity-sensitiveelectro-optic displays, a minimum of three driver voltage levels arenecessary. The three driver voltages required are V−, which drives apixel negative with respect to the top plane voltage, V+, which drives apixel positive with respect to the top plane voltage, and 0 V withrespect to the top plane voltage, which will hold the pixel in the samedisplay state.

The methods of the present invention can, however, be practiced withthis type of conventional LCD driver, provided that the controller isarranged to apply an appropriate sequence of voltages to the inputs ofone or more column drivers, and their associated row drivers, in orderto apply the necessary impulses to the pixels of an electro-opticdisplay.

There are two principal variants of this approach. In the first variant,all the impulses applied must have one of three values: +I, −I or 0,where:+I=−(−I)=Vapp*t _(pulse)where Vapp is the applied voltage above the top plane voltage, andt_(pulse) is the pulse length in seconds. This variant only allows thedisplay to operate in a binary (black/white) mode. In the secondvariant, the applied impulses may vary from +I to −I, but must beintegral multiples of Vapp/freq, where freq is the refresh frequency ofthe display.

This variant takes advantage of the fact that, as already noted,conventional LCD drivers are designed to reverse polarity at frequentintervals to avoid certain undesirable effects which might otherwise beproduced in the display. Consequently, such drivers are arranged toreceive from the controller a polarity or control voltage, which caneither be high or low. When a low control voltage is asserted, theoutput voltage on any given driver output line can adopt one of two outof the possible three voltages required, say V1 or V2, while when a highcontrol voltage is asserted, the output voltage on any given line canadopt one of a different two of the possible three voltages required,say V2 or V3. Thus, while only two out of the three required voltagescan be addressed at any specific time, all three voltages can beachieved at differing times. The three required voltages will usuallysatisfy the relationship:V2=(V3+V1)/2and V1 may be at or near the logic ground.

In this method, the display will be scanned 2*tpulse*freq times. Forhalf these scans (i.e., for tpulse*freq scans), the driver will be setto output either V1 or V2, which will normally be equal to −V and Vcom,respectively. Thus, during these scans, the pixels are either drivennegative, or held in the same display state. For the other half of thescans, the driver will be switched to output either V2 or V3, which willnormally be at Vcom and +V respectively. In these scans, the pixels aredriven positive or held in the same display state. Table 2 belowillustrates how these options can be combined to produce a drive ineither direction or a hold state; the correlation of positive drivingwith approach to a dark state and negative driving with approach to alight state is of course a function of the specific electro-optic mediumused.

TABLE 2 Drive sequence for achieving bi-directional drive plus hold withSTN drivers Driver outputs Desired Drive V1-V2 V2-V3 positive (drivedark) V2 V3 negative (drive white) V1 V2 hold V2 V2

There are several different ways to arrange the two portions of thedrive scheme (i.e., the two different types of scans or “frames”). Forexample, the two types of frames could alternate. If this is done at ahigh refresh rate, then the electro-optic medium will appear to besimultaneously lightening and darkening, when in fact it is being drivenin opposed direction in alternate frames. Alternatively, all of theframes of one type could occur before any of the frames of the secondtype; this would result in a two-step drive appearance. Otherarrangements are of course possible; for example two or more frames ofone type followed by two or more of the opposed type. Additionally, ifthere are no pixels that need to be driven in one of the two directions,then the frames of that polarity can be dropped, reducing the drive timeby 50%.

While this first variant can only produce binary images, the secondvariant can render images with multiple gray scale levels. This isaccomplished by combining the drive scheme described above withmodulation of the pulse widths for different pixels. In this case, thedisplay is again scanned 2*tpulse*freq times, but the driving voltage isonly applied to any particular pixel during enough of these scans toensure that the desired impulse for that particular pixel is achieved.For example, for each pixel, the total applied impulse could berecorded, and when the pixel reached its desired impulse, the pixelcould be held at the top plane voltage for all subsequent scans. Forpixels that need to be driven for less than the total scanning time, thedriving portion of this time (i.e., the portion of the time during whichan impulse is applied to change the display state of the pixel, asopposed to the holding portion during which the applied voltage simplymaintains the display state of the pixel) may be distributed in avariety of ways within the total time. For example, all driving portionscould be set to start at the beginning of the total time, or all drivingportions could instead be timed to complete at the end of the totaltime. As in the first variant, if at any time in the second variant nofurther impulses of a particular polarity need to be applied to anypixel, then the scans applying pulses of that polarity can beeliminated. This may mean that the entire pulse is shortened, forexample, if the maximum impulse to be applied in both the positive andnegative directions is less than the maximum allowable impulse.

To take a highly simplified case for purposes of illustration, considerthe application of the gray scale scheme described above to a displayhaving four gray levels, namely black (level 0), dark gray (level 1),light gray (level 2) and white (level 3). One possible drive scheme forsuch a display is summarized in Table 3 below.

TABLE 3 Frame No. 1 2 3 4 5 6 Parity Odd Even Odd Even Odd EvenTransition 0-3 + 0 + 0 + 0 0-2 + 0 + 0 0 0 0-1 + 0 0 0 0 0 0-0 0 0 0 0 00 3-0 0 − 0 − 0 − 2-0 0 − 0 − 0 0 1-0 0 − 0 0 0 0

For ease of illustration, this drive scheme is assumed to use only sixframes, although in practice a greater number would typically beemployed. These frames are alternately odd and even. White-goingtransitions (i.e., transitions in which the gray level is increased) aredriven only on the odd frames, while black-going transitions (i.e.,transitions in which the gray level is decreased) are driven only on theeven frames. On any frame when a pixel is not being driven, it is heldat the same voltage as the common front electrode, as indicated by “0”in Table 3. For the 0-3 (black-white) transition, a white-going impulseis applied (i.e., the pixel electrode is held at a voltage relative tothe common front electrode which tends to increase the gray level of thepixel) in each of the odd frames, Frames 1, 3 and 5. For a 0-2 (black tolight gray) transition, on the other hand, a white-going impulse isapplied only in Frames 1 and 3, and no impulse is applied in Frame 5;this is of course arbitrary, and, for example, a white-going impulsecould be applied in Frames 1 and 5 and no impulse applied in Frame 3.For a 0-1 (black to dark gray) transition, a white-going impulse isapplied only in Frame 1, and no impulse is applied in Frames 3 and 5;again, this is arbitrary, and, for example, a white-going impulse couldbe applied in Frame 3 and no impulse applied in Frames 1 and 5.

The black-going transitions are handled in a manner exactly similar tothe corresponding white-going transitions except that the black-goingimpulses are applied only in the even frames of the drive scheme. It isbelieved that those skilled in driving electro-optic displays willreadily be able to understand the manner in which the transitions notshown in Table 3 are handled from the foregoing description.

The sets of impulses described above can either be stand-alonetransitions between two images (as in general image flow), or they maybe part of a sequence of impulses designed to accomplish an imagetransition (as in a slide-show waveform, as discussed in more detailbelow).

Although emphasis has been laid above on driving methods which permitthe use of conventional drivers designed for use with LCD's, the presentmethods can make use of custom drivers, and a driver which is intendedto enable accurate control of gray states in an electro-optic display,while achieving rapid writing of the display will now be described withreference to FIGS. 6 and 7.

As already discussed, to first order, many electro-optic media respondto a voltage impulse, which can be expressed as V times t (or moregenerally, by the integral of V with respect to t) where V is thevoltage applied to a pixel and t is the time over which the voltage isapplied. Thus, gray states can be obtained by modulating the length ofthe voltage pulse applied to the display, or by modulating the appliedvoltage, or by a combination of these two.

In the case of pulse width modulation in an active matrix display, theattainable pulse width resolution is simply the inverse of the refreshrate of the display. In other words, for a display with a 100 Hz refreshrate, the pulse length can be subdivided into 10 ms intervals. This isbecause each pixel in the display is only addressed once per scan, whenthe select line for the pixels in that row are activated. For the restof the time, the voltage on the pixel may be maintained by a storagecapacitor, as described in the aforementioned WO 01/07961. As theresponse speed of the electro-optic medium becomes faster, the slope ofthe reflectivity versus time curve becomes steeper and steeper. Thus, tomaintain the same gray scale resolution, the refresh rate of the displaymust increase accordingly. Increasing the refresh rate results in higherpower consumption, and eventually becomes impractical as the transistorsand drivers are expected to charge the pixel and line capacitance in ashorter and shorter time.

On the other hand, in a voltage modulated display, the impulseresolution is simply determined by the number of voltage steps, and isindependent of the speed of the electro-optic medium. The effectiveresolution can be increased by imposing a nonlinear spacing of thevoltage steps, concentrating them where the voltage/reflectivityresponse of the electro-optic medium is steepest.

FIG. 6 of the accompanying drawings is a schematic representation of thetradeoffs between the pulse width modulation (PWM) and voltagemodulation (VM) approaches. The horizontal axis represents pulse length,while the vertical axis represents voltage. The reflectivity of theparticle-based electrophoretic display as a function of these twoparameters is represented as a contour plot, with the bands and spacesrepresenting differences of 1 L* in the reflected luminance of thedisplay. (It has been found empirically that a difference in luminanceof 1 L* is just noticeable to an average subject in dual stimulusexperiments.) The particular particle-based electrophoretic medium usedin the experiments summarized in FIG. 6 had a response time of 200 ms atthe maximum voltage (16 V) shown in the Figure.

The effects of pulse width modulation alone can be determined bytraversing the plot horizontally along the top, while the effects ofvoltage modulation alone are seen by examining the right vertical edge.From this plot, it is clear that, if a display using this particularmedium were driven at a refresh rate of 100 Hz in a pulse widthmodulation (PWM) mode, it would not be possible to obtain a reflectivitywithin ±1 L* in the middle gray region, where the contours are steepest.In voltage modulation (VM) mode, achieving a reflectivity within ±1 L*would require 128 equally spaced voltage levels, while running at aframe rate as low as 5 Hz (assuming, of course, that the voltage holdingcapability of the pixel, provided by a capacitor, is high enough). Inaddition, these two approaches can be combined to achieve the sameaccuracy with fewer voltage levels. To further reduce the requirednumber of voltage levels, they could be concentrated in the steep middleportion of the curves shown in FIG. 6 but made sparse in the outerregions. This could be accomplished with a small number of input gammavoltages. To further reduce the required number of voltage levels, theycould be concentrated at advantageous values. For example, very smallvoltages are not useful for achieving transitions if application of sucha small voltage over the allotted address time is not sufficient to makeany of the desired gray state transitions. Choosing a distribution ofvoltages that excludes such small voltages allows the allowed voltagesto be more advantageously placed.

Since bistable electro-optic displays are sensitive to the polarity ofthe applied electric field, as noted above, it is not desirable toreverse the polarity of the drive voltage on successive frames (images),as is usually done with LCD's, and frame, pixel and line inversion areunnecessary, and indeed counterproductive. For example, LCD drivers withpixel inversion deliver voltages of alternating polarity in alternateframes. Thus, it is only possible to deliver an impulse of the properpolarity in one half of the frames. This is not a problem in an LCD,where the liquid crystal material in not sensitive to polarity, but in abistable electro-optic display it doubles the time required to addressthe electro-optic medium.

Similarly, because bistable electro-optic displays are impulsetransducers and not voltage transducers, the displays integrate voltageerrors over time, which can result in large deviations of the pixels ofthe display from their desired optical states. This makes it importantto use drivers with high voltage accuracy, and a tolerance of ±3 mV orless is recommended.

To enable a driver to address a monochrome XGA (1024×768) display panelat a 75 Hz refresh rate, a maximum pixel clock rate of 60 MHz isrequired; achieving this clock rate is within the state of the art.

As already mentioned, one of the primary virtues of particle-basedelectrophoretic and other similar bistable electro-optic displays istheir image stability, and the consequent opportunity to run the displayat very low power consumption. To take maximum advantage of thisopportunity, power to the driver should be disabled when the image isnot changing. Accordingly, the driver should be designed to power downin a controlled manner, without creating any spurious voltages on theoutput lines. Because entering and leaving such a “sleep” mode will be acommon occurrence, the power-down and power-up sequences should be asrapid as possible, and should have minimal effects on the lifetime ofthe driver.

In addition, there should be an input pin that brings all of the driveroutput pins to Vcom, which will hold all of the pixels at their currentoptical state without powering down the driver.

The present drivers are useful, inter alia, for driving medium to highresolution, high information content portable displays, for example a 7inch (178 mm) diagonal XGA monochrome display. To minimize the number ofintegrated circuits required in such high resolution panels, it isdesirable to use drivers with a high number (for example, 324) ofoutputs per package. It is also desirable that the driver have an optionto run in one or more other modes with fewer of its outputs enabled. Thepreferred method for attaching the integrated circuits to the displaypanel is tape carrier package (TCP), so it is desirable to arrange thesizing and spacing of the driver outputs to facilitate use of thismethod.

The present drivers will typically be used to drive small to medium sizeactive matrix panels at around 10-30 V. Accordingly, the drivers shouldbe capable of driving capacitative loads of approximately 100 pF.

A block diagram of a preferred driver (generally designated 200) usefulin the methods of the invention is given in FIG. 7 of the accompanyingdrawings. This driver 200 comprises a shift register 202, a dataregister 204, a data latch 206, a digital to analogue converter (DAC)208 and an output buffer 210. This driver differs from those typicallyused to drive LCD's in that it provides for a polarity bit associatedwith each pixel of the display, and for generating an output above orbelow the top plane voltage controlled by the relevant polarity bit.

The signal descriptions for this preferred driver are given in thefollowing Table 4:

TABLE 4 Symbol Pin Name Description VDD Logic power supply 2.7-3.6 VAVDD Driver power supply 10-30 V VSS Ground 0 V Y1-Y324 Driver outputs,fed to the D/A converted 64 level column electrodes of the analogoutputs display D0(0:5) Display data input, odd dots 6 bit gray scaledata for odd dots, D0:0 = least significant bit (LSB) D1(0:5) Displaydata input, even 6 bit gray scale data for dots even dots, D1:0 = LSBD0POL Odd dot polarity control Determines which set of input gammavoltages current odd dot will reference. D0POL = 1: odd dot willreference VGAM6-11 D0POL = 0: odd dot will reference VGAM1-6 D1POL Evendot polarity control Determines which set of input gamma voltagescurrent even dot will reference. D1POL = 1: odd dot will referenceVGAM6-11 D1POL = 0: odd dot will reference VGAM1-6 SHL Shift directioncontrol input Controls shift direction in 162 bit shift register SHL =H: DIO1 input, Y1->Y324 SHL = L: DIO1 output, Y324->Y1 DIO1 Start pulseinput/output SHL = H: Used as the start pulse input pin SHL = L: Used asthe start pulse output pin DIO2 Start pulse input/output for SHL = H:Used as the start 256 lines pulse output pin for 256 lines active SHL =L: Used as the start pulse input pin for 256 lines, tie low if not usedDIO3 Start pulse input/output for SHL = H: Used as the start 260 linespulse output pin for 260 lines active SHL = L: Used as the start pulseinput pin for 260 lines, tie low if not used DIO4 Start pulseinput/output for SHL = H: Used as the start 300 lines pulse output pinfor 300 lines active SHL = L: Used as the start pulse input pin for 300lines, tie low if not used DIO5 Start pulse input/output for SHL = H:Used as the start 304 lines pulse output pin for 304 lines active SHL =L: Used as the start pulse input pin for 304 lines, tie low if not usedDIO6 Start pulse input/output for SHL = H: Used as the start 320 linespulse output pin for 320 lines active SHL = L: Used as the start pulseinput pin for 320 lines, tie low if not used DIO7 Start pulseinput/output for SHL = H: Used as the start 324 lines pulse output pinfor 324 lines active SHL = L: Used as the start pulse input pin for 324lines, tie low if not used CLK1 Shift clock input Two 6 bit gray valuesand two polarity control values for two display dots are loaded at everyrising edge CLK2 Latch input Latches the contents of the data registeron a rising edge and transfers latched values to the D/A converterblock. BL Blanking input (this does Sets all outputs to VGAM6 notactually blank the level BL = H: All outputs bistable display, butsimply set to VGAM6 BL = L: All stops the driver writing to outputsreflect D/A values the display, thus allowing the image already writtento remain) VGAM1-6 Lower gamma reference Determine grayscale voltagesvoltage outputs through resistive DAC system VGAM6-11 Upper gammareference Determine grayscale voltages voltage outputs through resistiveDAC system

The driver 200 operates in the following manner. First, a start pulse isprovided by setting (say) DIO1 high to reset the shift register 202 to astarting location. (As will readily be apparent to those skilled indisplay driver technology, the various DIOx inputs to the shift registerare provided to enable the driver to be used with displays havingvarying numbers of columns, and only one of these inputs is used withany given display, the others being tied permanently low.) The shiftregister now operates in the conventional manner used in LCD's; at eachpulse of CLK1, one and only one of the 162 outputs of the shift register202 goes high, the others being held low, and the high output beingshifted one place at each pulse of CLK1. As schematically indicated inFIG. 7, each of the 162 outputs of the shift register 202 is connectedto two inputs of data register 204, one odd input and one even input.

The display controller (cf. FIG. 2) provides two six-bit impulse valuesD0(0:5) and D1(0:5) and two single-bit polarity signals D0POL and D1POLon the inputs of the data register 204. At the rising edge of each clockpulse CLK1, two seven-bit numbers (D0POL+D0(0:5) and D1POL+D1(0:5)) arewritten into registers in data register 204 associated with the selected(high) output of shift register 202. Thus, after 162 clock pulses CLK1,324 seven-bit numbers (corresponding to the impulse values for onecomplete line of the display for one frame) have been written into the324 registers present in data register 204.

At the rising edge of each clock pulse CLK2, these 324 seven-bit numbersare transferred from the data register 204 to the data latch 206. Thenumbers thus placed in the data latch 206 are read by the DAC 208 and,in conventional fashion, corresponding analogue values are placed on theoutputs of the DAC 208 and fed, via the buffer 210 to the columnelectrodes of the display, where they are applied to pixel electrodes ofone row selected in conventional fashion by a row driver (not shown). Itshould be noted, however, that the polarity of each column electrodewith respect to Vcom is controlled by the polarity bit D0POL or D1POLwritten into the data latch 206 and thus these polarities do notalternate between adjacent column electrodes in the conventional mannerused in LCD's.

FIG. 8 is a flow chart illustrating a program which may be run by thecontroller unit shown in FIGS. 1 and 2. This program (generallydesignated 300) is intended for use with a look-up table method(described in more detail below) in which all pixels of a display areerased and then re-addressed each time an image is written or refreshed.

The program begins with a “powering on” step 302 in which the controlleris initialized, typically as a result of user input, for example a userpushing the power button of a personal digital assistant (PDA). The step302 could also be triggered by, for example, the opening of the case ofa PDA (this opening being detected either by a mechanical sensor or by aphotodetector), by the removal of a stylus from its rest in a PDA, bydetection of motion when a user lifts a PDA, or by a proximity detectorwhich detects when a user's hand approaches a PDA.

The next step 304 is a “reset” step in which all the pixels of thedisplay are driven alternately to their black and white states. It hasbeen found that, in at least some electro-optic media, such “flashing”of the pixels is necessary to ensure accurate gray states during thesubsequent writing of an image on the display. It has also been foundthat typically at least five flashes (counting each successive black andwhite state as one flash) are required, and in some cases more. Thegreater the number of flashes, the more time and energy that this stepconsumes, and thus the longer the time that must elapse before the usercan see a desired image upon the display. Accordingly, it is desirablethat the number of flashes be kept as small as possible consistent withaccurate rendering of gray states in the image subsequently written. Atthe conclusion the reset step 304, all the pixels of the display are inthe same black or white state.

The next step 306 is a writing or “sending out image” step in which thecontroller 16 sends out signals to the row and column drivers 22 and 24respectively (FIGS. 1 and 2) in the manner already described, thuswriting a desired image on the display. Since the display is bistable,once the image has been written, it does not need to be rewrittenimmediately, and thus after writing the image, the controller can causethe row and column drivers to cease writing to the display, typically bysetting a blanking signal (such as setting signal BL in FIG. 7 high).

The controller now enters a decision loop formed by steps 308, 310 and312. In step 308, the controller 16 checks whether the computer 12(FIG. 1) requires display of a new image. If so, the controllerproceeds, in an erase step 314 to erase the image written to the displayat step 306, thus essentially returning the display to the state reachedat the end of reset step 304. From erase step 314, the controllerreturns to step 304, resets as previously described, and proceeds towrite the new image.

If at step 308 no new image needs to be written to the display, thecontroller proceeds to a step 310, at which it determines when the imagehas remained on the display for more than a predetermined period. As iswell known to those skilled in display technology, images written onbistable media do not persist indefinitely, and the images graduallyfade (i.e., lose contrast). Furthermore, in some types of electro-opticmedium, especially electrophoretic media, there is often a trade-offbetween writing speed of the medium and bistability, in that media whichare bistable for hours or days have substantially longer writing timesthan media which are only bistable for seconds or minutes. Accordingly,although it is not necessary to rewrite the electro-optic mediumcontinuously, as in the case of LCD's, to provide an image with goodcontrast, it may be desirable to refresh the image at intervals of (say)a few minutes. Thus, at step 310 the controller determines whether thetime which has elapsed since the image was written at step 306 exceedssome predetermined refresh interval, and if so the controller proceedsto erase step 314 and then to reset step 304, resets the display aspreviously described, and proceeds to rewrite the same image to thedisplay.

(The program shown in FIG. 8 may be modified to make use of both localand global rewriting. If so, step 310 may be modified to decide whetherlocal or global rewriting is required. If, in this modified program, atstep 310 the program determines that the predetermined time has notexpired, no action is taken. If, however, the predetermined time hasexpired, step 310 does not immediately invoke erasure and rewriting ofthe image; instead step 310 simply sets a flag (in the normal computersense of that term) indicating that the next image update should beeffected globally rather than locally. The next time the program reachesstep 306, the flag is checked; if the flag is set, the image isrewritten globally and then the flag is cleared, but if the flag is notset, only local rewriting of the image is effected.)

If at step 310 it is determined that the refresh interval has not beenexceeded, the controller proceeds to a step 312, where it determineswhether it is time to shut down the display and/or the image source. Inorder to conserve energy in a portable apparatus, the controller willnot allow a single image to be refreshed indefinitely, and terminatesthe program shown in FIG. 8 after a prolonged period of inactivity.Accordingly, at step 310 the controller determines whether apredetermined “shut-down” period (greater than the refresh intervalmentioned above) has elapsed since a new image (rather than a refresh ofa previous image) was written to the display, and if so the programterminates, as indicated at 314. Step 314 may include powering down theimage source. Naturally, the user still has access to a slowly-fadingimage on the display after such program termination. If the shut-downperiod has not been exceeded, the controller proceeds from step 312 backto step 308.

Some general considerations regarding waveforms to be used in themethods of the present invention will be discussed.

Waveforms for bistable displays that exhibit the aforementioned memoryeffect can be grouped into two major classes, namely compensated anduncompensated. In a compensated waveform, all of the pulses areprecisely adjusted to account for any memory effect in the pixel. Forexample, a pixel undergoing a series of transitions through gray scalelevels 1-3-4-2 might receive a slightly different impulse for the 4-2transition than a pixel that undergoes a transition series 1-2-4-2. Suchimpulse compensation could occur by adjusting the pulse length, thevoltage, or by otherwise changing the V(t) profile of the pulses. In anuncompensated waveform, no attempt is made to account for any priorstate information (other than the initial state). In an uncompensatedwaveform, all pixels undergoing the 4-2 transition would receiveprecisely the same pulse. For an uncompensated waveform to worksuccessfully, one of two criteria must be met. Either the electro-opticmaterial must not exhibit a memory effect in its switching behavior, oreach transition must effectively eliminate any memory effect on thepixel.

In general, uncompensated waveforms are best suited for use with systemscapable of only coarse impulse resolution. Examples would be a displaywith tri-level drivers, or a display capable of only 2-3 bits of voltagemodulation. A compensated waveform requires fine impulse adjustments,which are not possible with these systems. Obviously, while acoarse-impulse system is preferably restricted to uncompensatedwaveforms, a system with fine impulse adjustment can implement eithertype of waveform.

The simplest uncompensated waveform is 1-bit general image flow (1-bitGIF). In 1-bit GIF, the display transitions smoothly from one pureblack-and-white image to the next. The transition rule for this sequencecan be stated simply: if a pixel is switching from white to black, thenapply an impulse I. If it is switching from black to white, apply theimpulse of the opposite polarity, −I. If a pixel remains in the samestate, then no impulse is applied to that pixel. As previously stated,the mapping of the impulse polarity to the voltage polarity of thesystem will depend upon the response function of the material.

Another uncompensated waveform that is capable of producing grayscaleimages is the uncompensated n-prepulse slide show (n-PP SS). Theuncompensated slide show waveform has three basic sections. First, thepixels are erased to a uniform optical state, typically either white orblack. Next, the pixels are driven back and forth between two opticalstates, again typically white and black. Finally, the pixel is addressedto a new optical state, which may be one of several gray states. Thefinal (or writing) pulse is referred to as the addressing pulse, and theother pulses (the first (or erasing) pulse and the intervening (orblanking) pulses) are collectively referred to as prepulses. A waveformof this type will be described below with reference to FIGS. 9 and 10.

Prepulse slide show waveforms can be divided into two basic forms, thosewith an odd number of prepulses, and those with an even number ofprepulses. For the odd-prepulse case, the erasing pulse may be equal inimpulse and opposite in polarity to the immediately previous writingpulse (again, see FIG. 9 and discussion thereof below). In other words,if the pixel is written to gray from black, the erasing pulse will takethe pixel back to the black state. In the even-prepulse case, theerasing pulse should be of the same polarity as the previous writingpulse, and the sum of the impulses of the previous writing pulse and theerasing pulse should be equal to the impulse necessary to fullytransition from black to white. In other words, if a pixel is writtenfrom black in the even-prepulse case, then it must be erased to white.

After the erasing pulse, the waveform includes either zero or an evennumber of blanking pulses. These blanking pulses are typically pulses ofequal impulse and opposite polarity, arranged so that the first pulse isof opposite polarity to the erasing pulse. These pulses will generallybe equal in impulse to a full black-white pulse, but this is notnecessarily the case. It is also only necessary that pairs of pulseshave equal and opposite impulses it is possible that there may be pairsof widely varying impulses chained together, i.e. +I, −I, +0.1I, −0.1I,+4I, −4I.

The last pulse to be applied is the writing pulse. The impulse of thispulse is chosen based only upon the desired optical state (not upon thecurrent state, or any prior state). In general, but not necessarily, thepulse will increase or decrease monotonically with gray state value.Since this waveform is specifically designed for use with coarse impulsesystems, the choice of the writing pulse will generally involve mappinga set of desired gray states onto a small number of possible impulsechoices, e.g. 4 gray states onto 9 possible applied impulses.

Examination of either the even or odd form of the uncompensatedn-prepulse slide show waveform will reveal that the writing pulse alwaysbegins from the same direction, i.e. either from black or from white.This is an important feature of this waveform. Since the principle ofthe uncompensated waveform is that the pulse length cannot becompensated accurately to ensure that pixels reach the same opticalstate, one cannot to expect to reach an identical optical state whenapproaching from opposite extreme optical states (black or white).Accordingly, there are two possible polarities for either of theseforms, which can be labeled “from black” and “from white.”

One major shortcoming of this type of waveform is that it haslarge-amplitude optical flashes between images. This can be improved byshifting the update sequence by one superframe time for half of thepixels, and interleaving the pixels at high resolution, as discussedbelow with reference to FIGS. 9 and 10. Possible patterns include everyother row, every other column, or a checkerboard pattern. Note, thisdoes not mean using the opposite polarity, i.e. “from black” versus“from white”, since this would result in non-matching gray scales onneighboring pixels. Instead, this can be accomplished by delaying thestart of the update by one “superframe” (a grouping of frames equivalentto the maximum length of a black-white update) for half of the pixels(i.e. the first set of pixels completes the erase pulse, then the secondset of pixels begin the erase pulse as the first set of pixels begin thefirst blanking pulse). This will require the addition of one superframefor the total update time, to allow for this synchronization.

Limited Transitions Method of the Present Invention

To avoid the aforementioned flashing problems of the drive schemes shownin FIGS. 9 and 10, while also avoiding the problems of general grayscaleimage flow previously discussed, it is advantageous, in accordance withthe limited transitions method of the present invention, to arrange thedrive scheme so that any given pixel can only undergo a predeterminedmaximum number (at least one) of gray scale transitions before passingthrough one extreme optical state (black or white). A transition awayfrom the extreme optical state start from an accurately known opticalstate, in effect canceling out any previously accumulated errors.Various techniques for minimizing the optical effects of such passage ofpixels through extreme optical states (such as flashing of the display)are discussed below.

Before describing the limited transitions method of the presentinvention in detail, other ways of reducing the flashing problem willfirst be described. A first, simple drive scheme will now be describedwith reference to a simple two-bit gray scale system having black (level0), dark gray (level 1), light gray (level 2) and white (level 3)optical states, transitions being effected using a pulse widthmodulation technique, and a look-up table for transitions as set out inTable 5 below.

TABLE 5 Transition Impulse Transition Impulse 0-0 0 0-0 0 0-1 n 1-0 −n0-2 2n 2-0 −2n 0-3 3n 3-0 −3nwhere n is a number dependent upon the specific display, and −nindicates a pulse having the same length as a pulse n but of oppositepolarity. It will further be assumed that at the end of the reset pulse304 in FIG. 8 all the pixels of the display are black (level 0). Since,as described below, all transitions take place through an interveningblack state, the only transitions effected are those to or from thisblack state. Thus, the size of the necessary look-up table issignificantly reduced, and obviously the factor by which look-up tablesize is thus reduced increases with the number of gray levels of thedisplay.

FIG. 9 shows the transitions of one pixel associated with the drivescheme of FIG. 8. At the beginning of the reset step 304, the pixel isin some arbitrary gray state. During the reset step 304, the pixel isdriven alternately to three black states and two intervening whitestates, ending in its black state. The pixel is then, at 306, writtenwith the appropriate gray level for a first image, assumed to belevel 1. The pixel remains at this level for some time during which thesame image is displayed; the length of this display period is greatlyreduced in FIG. 9 for ease of illustration. At some point, a new imageneeds to be written, and at this point, the pixel is returned to black(level 0) in erase step 308, and is then subjected, in a second resetstep designated 304′, to six reset pulses, alternately white and black,so that at the end of this reset step 304′, the pixel has returned toits black state. Finally, in a second writing step designated 306′, thepixel is written with the appropriate gray level for a second image,assumed to be level 2.

Numerous variations of the drive scheme shown in FIG. 9 are of coursepossible. One useful variation is shown in FIG. 10. The steps 304, 306and 308 shown in FIG. 10 are identical to those shown in FIG. 9.However, in step 304′, five reset pulses are used (obviously a differentodd number of pulses could also be used), so that at the end of step304′, the pixel is in its white state (level 3), and in the secondwriting step 306′, writing of the pixel is effected from this whitestate rather than the black state as in FIG. 9. Successive images arethen written alternately from black and white states of the pixel.

In a further variation of the drive schemes shown in FIGS. 9 and 10,erase step 308 is effected to as to drive the pixel white (level 3)rather than black. An even number of reset pulses are then applied tothat the pixel ends the reset step in a white state, and the secondimage is written from this white state. As with the drive scheme shownin FIG. 10, in this scheme successive images are written alternatelyfrom black and white states of the pixel.

It will be appreciated that in all the foregoing schemes, the number andduration of the reset pulses can be varied depending upon thecharacteristics of the electro-optic medium used. Similarly, voltagemodulation rather than pulse width modulation may be used to vary theimpulse applied to the pixel.

The black and white flashes which appear on the display during the resetsteps of the drive schemes described above are of course visible to theuser and may be objectionable to many users. To lessen the visual effectof such reset steps, it is convenient to divide the pixels of thedisplay into two (or more) groups and to apply different types of resetpulses to the different groups. More specifically, if it necessary touse reset pulses which drive any given pixel alternately black andwhite, it is convenient to divide the pixels into at least two groupsand to arrange the drive scheme so that one group of pixels are drivenwhite at the same time that another group are driven black. Provided thespatial distribution of the two groups is chosen carefully and thepixels are sufficiently small, the user will experience the reset stepas an interval of gray on the display (with perhaps some slightflicker), and such a gray interval is typically less objectionable thana series of black and white flashes.

For example, in one form of such a “two group reset” step, the pixel inodd-numbered columns may be assigned to one “odd” group and the pixelsin the even-numbered columns to the second “even” group. The odd pixelscould then make use of the drive scheme shown in FIG. 9, while the evenpixels could make use of a variant of this drive scheme in which, duringthe erase step, the pixels are driven to a white rather a black state.Both groups of pixels would then be subjected to an even number of resetpulses during reset step 304′, so that the reset pulses for the twogroups are essentially 180° out of phase, and the display appears graythroughout this reset step. Finally, during the writing of the secondimage at step 306′, the odd pixels are driven from black to their finalstate, while the even pixels are driven from white to their final state.In order to ensure that every pixel is reset in the same manner over thelong term (and thus that the manner of resetting does not introduce anyartifacts on to the display), it is advantageous for the controller toswitch the drive schemes between successive images, so that as a seriesof new images are written to the display, each pixel is written to itsfinal state alternately from black and white states.

Obviously, a similar scheme can be used in which the pixels inodd-numbered rows form the first group and the pixels in even-numberedrows the second group. In a further similar drive scheme, the firstgroup comprises pixels in odd-numbered columns and odd-numbered rows,and even-numbered columns and even-numbered rows, while the second groupcomprises in odd-numbered columns and even-numbered rows, andeven-numbered columns and odd-numbered rows, so that the two groups aredisposed in a checkerboard fashion.

Instead of or in addition to dividing the pixels into two groups andarranging for the reset pulses in one group to be 180° out of phase withthose of the other group, the pixels may be divided into groups whichuse different reset steps differing in number and frequency of pulses.For example, one group could use the six pulse reset sequence shown inFIG. 9, while the second could use a similar sequence having twelvepulses of twice the frequency. In a more elaborate scheme, the pixelscould be divided into four groups, with the first and second groupsusing the six pulse scheme but 180° out of phase with each other, whilethe third and fourth groups use the twelve pulse scheme but 180° out ofphase with each other.

In accordance with the limited transitions method of the presentinvention, further reductions in flashing problems may be effected byusing a drive scheme which permits any given to assume a non-zero butlimited number of successive gray states before touching an opticalrail. In such a drive scheme, when the display is rewritten to display anew image thereon, any pixel, which has undergone a number oftransitions exceeding a predetermined value without touching an extremeoptical state, is driven to at least one extreme optical state beforedriving that pixel to its final optical state. In a preferred form ofsuch a drive scheme, a pixel driven to an extreme optical state isdriven to the extreme optical state which is closer in gray level to theoptical state desired after the transition, assuming of course that thisdesired optical state is not one of the extreme optical states. Also, ina preferred form of such a drive scheme using a look-up table aspreviously described, the maximum number of transitions which a pixel isallowed to undergo without touching an optical rail (extreme opticalstate) is set equal to the number of prior optical states taken intoaccount in the transition matrix; such a method requires no extracontroller logic or memory.

Driving methods which limit the maximum number of transitions beforetouching an optical rail need not significantly increase the time takenfor a complete rewriting of the display. For example, consider a fourgray level (2 bit) display in which a transition from white to black orvice versa takes 200 msec, so that a general grayscale image flow drivescheme takes this time to completely rewrite the display. The only casewhere a transition needs to be modified in such a display is when apixel is repeatedly toggled between the two central gray levels. If sucha pixel is toggled between the two central gray levels for a number oftransitions which exceeds the predetermined number, the limitedtransitions method of the present invention requires that the nexttoggling be effected via one optical rail (extreme optical state). Ithas been found that in such a case the transition to the optical railtakes about 70 msec, while the subsequent transition to the gray leveltakes about 130 msec, so that the total transition time is only about200 msec. Thus, the present limited transitions method does not requireany increase in transition time as compared with general grayscale imageflow.

A limited transitions drive method which reduces the objectionableeffects of reset steps will now be described with reference to FIGS. 11Aand 11B. In this scheme, the pixels are again divided into two groups,with the first (even) group following the drive scheme shown in FIG. 11Aand the second (odd) group following the drive scheme shown in FIG. 11B.Also in this scheme, all the gray levels intermediate black and whiteare divided into a first group of contiguous dark gray levels adjacentthe black level, and a second group of contiguous light gray levelsadjacent the white level, this division being the same for both groupsof pixels. Desirably but not essentially, there are the same number ofgray levels in these two groups; if there are an odd number of graylevels, the central level may be arbitrarily assigned to either group.For ease of illustration, FIGS. 11A and 11B show this drive schemeapplied to an eight-level gray scale display, the levels beingdesignated 0 (black) to 7 (white); gray levels 1, 2 and 3 are dark graylevels and gray levels 4, 5 and 6 are light gray levels.

In the drive scheme of FIGS. 11A and 11B, gray to gray transitions arehandled according to the following rules:

(a) in the first, even group of pixels, in a transition to a dark graylevel, the last pulse applied is always a white-going pulse (i.e., apulse having a polarity which tends to drive the pixel from its blackstate to its white state), whereas in a transition to a light graylevel, the last pulse applied is always a black-going pulse;

(b) in the second, odd group of pixels, in a transition to a dark graylevel, the last pulse applied is always a black-going pulse, whereas ina transition to a light gray level, the last pulse applied is always awhite-going pulse;

(c) in all cases, a black-going pulse may only succeed a white-goingpulse after a white state has been attained, and a white-going pulse mayonly succeed a black-going pulse after a black state has been attained;and

(d) even pixels may not be driven from a dark gray level to black by asingle black-going pulse nor odd pixels from a light gray level to whiteusing a single white-going pulse.

(Obviously, in all cases, a white state can only be achieved using afinal white-going pulse and a black state can only be achieved using afinal black-going pulse.)

The application of these rules allows each gray to gray transition to beeffected using a maximum of three successive pulses. For example, FIG.11A shows an even pixel undergoing a transition from black (level 0) togray level 1. This is achieved with a single white-going pulse (shown ofcourse with a positive gradient in FIG. 11A) designated 1102. Next, thepixel is driven to gray level 3. Since gray level 3 is a dark graylevel, according to rule (a) it must be reached by a white-going pulse,and the level 1/level 3 transition can thus be handled by a singlewhite-going pulse 1104, which has an impulse different from that ofpulse 1102.

The pixel is now driven to gray level 6. Since this is a light graylevel, it must, by rule (a) be reached by a black-going pulse.Accordingly, application of rules (a) and (c) requires that this level3/level 6 transition be effected by a two-pulse sequence, namely a firstwhite-going pulse 1106, which drives the pixel white (level 7), followedby a second black-going pulse 1108, which drives the pixel from level 7to the desired level 6.

The pixel is next driven to gray level 4. Since this is a light graylevel, by an argument exactly similar to that employed for the level1/level 3 transition discussed earlier, the level 6/level 4 transitionis effected by a single black-going pulse 1110. The next transition isto level 3. Since this is a dark gray level, by an argument exactlysimilar to that employed for the level 3/level 6 transition discussedearlier, the level 4/level 3 transition is handled by a two-pulsesequence, namely a first black-going pulse 1112, which drives the pixelblack (level 0), followed by a second white-going pulse 1114, whichdrives the pixels from level 0 to the desired level 3.

The final transition shown in FIG. 11A is from level 3 to level 1. Sincelevel 1 is a dark gray level, it must, according to rule (a) beapproached by a white-going pulse. Accordingly, applying rules (a) and(c), the level 3/level 1 transition must be handled by a three-pulsesequence comprising a first white-going pulse 1116, which drives thepixel white (level 7), a second black-going pulse 1118, which drives thepixel black (level 0), and a third white-going pulse 1120, which drivesthe pixel from black to the desired level 1 state.

FIG. 11B shows an odd pixel effecting the same 0-1-3-6-4-3-1 sequence ofgray states as the even pixel in FIG. 11A. It will be seen, however,that the pulse sequences employed are very different. Rule (b) requiresthat level 1, a dark gray level, be approached by a black-going pulse.Hence, the 0-1 transition is effected by a first white-going pulse 1122,which drives the pixel white (level 7), followed by a black-going pulse1124, which drives the pixel from level 7 to the desired level 1. The1-3 transition requires a three-pulse sequence, a first black-goingpulse 1126, which drives the pixel black (level 0), a second white-goingpulse 1128, which drives the pixel white (level 7), and a thirdblack-going pulse 1130, which drives the pixel from level 7 to thedesired level 3. The next transition is to level 6 is a light graylevel, which according to rule (b) is approached by a white-going pulse,the level 3/level 6 transition is effected by a two-pulse sequencecomprising a black-going pulse 1132, which drives the pixel black (level0), and a white-going pulse 1134, which drives the pixel to the desiredlevel 6. The level 6/level 4 transition is effected by a three-pulsesequence, namely a white-going pulse 1136, which drives the pixel white(level 7), a black-going pulse 1138, which drives the pixel black (level0) and a white-going pulse 1140, which drives the pixel to the desiredlevel 4. The level 4/level transition 3 transition is effected by atwo-pulse sequence comprising a white-going pulse 1142, which drives thepixel white (level 7), followed by a black-going pulse 1144, whichdrives the pixel to the desired level 3. Finally, the level 3/level 1transition is effected by a single black-going pulse 1146.

It will be seen from FIGS. 11A and 11B that this drive scheme ensuresthat each pixel follows a “sawtooth” pattern in which the pixel travelsfrom black to white without change of direction (although obviously thepixel may rest at any intermediate gray level for a short or longperiod), and thereafter travels from white to black without change ofdirection. Thus, rules (c) and (d) above may be replaced by a singlerule (e) as follows:

-   -   (e) once a pixel has been driven from one extreme optical state        (i.e., white or black) towards the opposed extreme optical state        by a pulse of one polarity, the pixel may not receive a pulse of        the opposed polarity until it has reached the aforesaid opposed        extreme optical state.

Thus, this drive scheme is a “rail-stabilized gray scale” or “RSGS”drive scheme. Such a RSGS drive scheme is a special case of a limitedtransitions drive scheme which ensures that a pixel can only undergo, atmost, a number of transitions equal to N/2 (or more accurately (N−1)/2)transitions, where N is the total number of gray levels capable of beingdisplayed, without requiring a transition to take place via an opticalrail. Such a drive scheme prevents slight errors in individualtransitions (caused, for example, by unavoidable minor fluctuations involtages applied by drivers) accumulating indefinitely to the pointwhere serious distortion of a gray scale image is apparent to anobserver. Furthermore, this drive scheme is designed so that even andodd pixels always approach a given intermediate gray level from opposeddirections, i.e., the final pulse of the sequence is white-going in onecase and black-going in the other. If a substantial area of the display,containing substantially equal numbers of even and odd pixels, is beingwritten to a single gray level, this “opposed directions” featureminimizes flashing of the area.

For reasons similar to those discussed above relating to other driveschemes which divide pixels into two discrete groups, when implementingthe sawtooth drive scheme of FIGS. 11A and 11B, careful attention shouldbe paid to the arrangements of the pixels in the even and odd groups.This arrangement will desirably ensure that any substantially contiguousarea of the display will contain a substantially equal number of odd andeven pixels, and that the maximum size of a contiguous block of pixelsof the same group is sufficiently small not to be readily discernable byan average observer. As already discussed, arranging the two groups ofpixels in a checkerboard pattern meets these requirements. Stochasticscreening techniques may also be employed to arrange the pixels of thetwo groups.

However, in this sawtooth drive scheme, use of a checkerboard patterntends to increase the energy consumption of the display. In any givencolumn of such a pattern, adjacent pixels will belong to oppositegroups, and in a contiguous area of substantial size in which all pixelsare undergoing the same gray level transition (a not uncommonsituation), the adjacent pixels will tend to require impulses ofopposite polarity at any given time. Applying impulses of oppositepolarity to consecutive pixels in any column requires discharging andrecharging the column (source) electrodes of the display as each newline is written. It is well known to those skilled in driving activematrix displays that discharging and recharging column electrodes is amajor factor in the energy consumption of a display. Hence, acheckerboard arrangement tends to increase the energy consumption of thedisplay.

A reasonable compromise between energy consumption and the desire toavoid large contiguous areas of pixels of the same group is to havepixels of each group assigned to rectangles, the pixels of which all liein the same column but extend for several pixels along that column. Withsuch an arrangement, when rewriting areas having the same gray level,discharging and recharging of the column electrodes will only benecessary when shifting from one rectangle to the next. Desirably, therectangles are 1×4 pixels, and are arranged so that rectangles inadjacent columns do not end on the same row, i.e., the rectangles inadjacent columns should have differing “phases”. The assignment ofrectangles in columns to phases may be effected either randomly or in acyclic manner.

One advantage of the sawtooth drive scheme shown in FIGS. 11A and 11B isthat any areas of the image which are monochrome are simply updated witha single pulse, either black to white or white to black, as part of theoverall updating of the display. The maximum time taken for rewritingsuch monochrome areas is only one-half of the maximum time for rewritingareas which require gray to gray transitions, and this feature can beused to advantage for rapid updating of image features such ascharacters input by a user, drop-down menus etc. The controller cancheck whether an image update requires any gray to gray transitions; ifnot, the areas of the image which need rewriting can be rewritten usingthe rapid monochrome update mode. Thus, a user can have fast updating ofinput characters, drop-down menus and other user-interaction features ofthe display seamlessly superimposed upon a slower updating of a generalgrayscale image.

FIGS. 43A and 43B illustrate respectively transitions occurring during aprior art drive scheme and a limited transitions method of the presentinvention, the latter being more general than that shown in FIGS. 11Aand 11B. As in FIGS. 11A and 11B, FIGS. 43A and 43B illustratetransitions occurring among 8 gray levels denoted 0 (black) to 7(white). The prior art method shown in FIG. 43A allows an unlimitednumber of transitions without touching an extreme optical state. FIG.43A illustrates a series of transitions 0-1-2-3-4-2-4-2-4-2-4-5-6-7. Inthe prior art method, extreme optical states 0 and 7 are achieved onlyat the beginning and end of the series; the intervening twelvetransitions are achieved without touching an extreme optical state.

FIG. 43B illustrates how the series of transitions shown in FIG. 43A ismodified using a limited transitions method of the present inventionwhich permits only three successive transitions without touching anextreme optical state. As shown in FIG. 43B, the actual transitionseffected are:

-   -   0-1-2-3-[0]-4-2-4-[7]-2-4-2-[0]-4-5-6-7        where numbers within brackets indicate an intermediate extreme        optical state inserted to limit the maximum number of        transitions without touching an extreme optical state to three.

A limited transitions drive scheme does not necessarily require the useof counters to measure the number of transitions undergone by each pixelof a display, and does not bar the use of drive schemes (such as thecyclic RSGS drive scheme already described with reference to FIGS. 11Aand 11B) which require certain transitions to take place via an opticalrail even if the predetermined number of transitions has not beenreached, provided that the algorithm used to determine the manner ofeffecting transitions does not permit any pixel to undergo more than thepredetermined number of transitions without touching an optical rail.Furthermore, it will be appreciated that the check on the number oftransitions undergone by a given pixel without touching an optical railneed not be made at every rewriting of the image on the display,especially in the case of displays (for example in watches) which areupdated at frequent intervals. For example, the check might be made ononly alternate updates, provided that all pixels which either exceededwith predetermined number of transitions or might exceed this numberafter the next update were driven to optical rails.

Another preferred limited transitions method of the invention will nowbe described, though by way of illustration only. This preferred methodis used to operate a four gray level (2 bit) active matrix display whichuses a transition matrix which takes account of only the initial andfinal gray levels (designated “R2” and “R1” respectively) of thetransition to be effected, and no additional prior states. The displaycontroller is a tri-level pulse width modulation (PWM) controllercapable of applying −V, 0 or +V to each pixel electrode relative to thecommon front electrode, which is held at 0.

The display controller contains two RAM image buffers. One buffer (“A”)stores the image currently on the display. Normally, the controller isin sleep mode, preserving the data in the RAM and keeping the displaydrivers inactive. The bistability of the electro-optic medium keeps thesame image on the display. When an image update command is received, thecontroller loads the new image into the second buffer (“B”). Then, foreach pixel of the display, the controller looks up (in FLASH memory) amulti-frame drive waveform, based on the desired final state R1 of thepixel (from buffer “B”) and the current, initial state R2 of each pixel(from buffer “A”).

The data in the flash memory file is organized as a three-dimensionalarray of voltage values, V(R1, R2, frame), where as already indicated R1and R2 are each integers from 1 to 4 (corresponding to the fouravailable gray levels), and “frame” is the frame number, i.e., thenumber of the relevant frame within the superframe used for eachtransition. Typically, the superframe might be 1 second long, with eachframe occupying 20 ms, so that the frame number can range from 1 to 50.Thus, the array has 4×4×50=800 entries. Since each entry in the arraymust be capable of representing any one of the voltage values −V, 0 and+V, typically two bits will be used to store each voltage value (arrayvalue).

It will immediately be apparent that, since each of the 800 arrayentries may have any one of the three possible voltage values, there area huge number of possible arrays (waveforms), the number being far toolarge to search exhaustively. In theory, there are 3800 or about 5×10381possible arrays; since there are about 1078 atoms in the universe and109 seconds in an average human lifetime, practical capabilities are atleast 200 orders of magnitude short of an exhaustive search.Fortunately, existing knowledge about the behavior of electro-opticdisplays, and especially the need for DC balance therein, imposeadditional constraints upon the possible waveforms and enable the searchfor an optimum or near optimum waveform to be confined withinpracticable limits.

As discussed in the aforementioned U.S. Pat. Nos. 6,504,524 and6,531,997 and the aforementioned 2003/0137521, it is known that most, ifnot all, electro-optic media require direct current (DC) balancedwaveforms, or deleterious effects may occur. Such effects may includedamage to electrodes and long term drift (over a period of hours) ofgray states over a range of several L* units when DC imbalancedwaveforms are used. Accordingly, it seems prudent to make every effortto use DC balanced drive wave schemes.

From what has been said above, it might at first appear that such DCbalancing may not be achievable, since the impulse, and thus the currentthrough the pixel, required for any particular gray to gray transitionis substantially constant. However, this is only true to a firstapproximation, and it has been found empirically that, at least in thecase of particle-based electrophoretic media (and the same appears to betrue of other electro-optic media), the effect of (say) applying fivespaced 50 msec pulses to a pixel is not the same as applying one 250msec pulse of the same voltage. Accordingly, there is some flexibilityin the current which is passed through a pixel to achieve a giventransition, and this flexibility can be used to assist in achieving DCbalance. For example, the look-up table can store multiple impulses fora given transition, together with a value for the total current providedby each of these impulses, and the controller can maintain, for eachpixel, a register arranged to store the algebraic sum of the impulsesapplied to the pixel since some prior time (for example, since the pixelwas last in a black state). When a specific pixel is to be driven from awhite or gray state to a black state, the controller can examine theregister associated with that pixel, determine the current required toDC balance the overall sequence of transitions from the previous blackstate to the forthcoming black state, and choose that one of themultiple stored impulses for the white/gray to black transition neededwhich will either accurately reduce the associated register to zero, orat least to as small a remainder as possible (in which case theassociated register will retain the value of this remainder and add itto the currents applied during later transitions). It will be apparentthat repeated applications of this process can achieve accurate longterm DC balancing of each pixel.

It is necessary to consider the precise definition of DC balance in awaveform. To determine if a waveform is DC balanced, a resistive modelof the electro-optic medium is normally used. Such a model is notcompletely accurate, but may be assumed to be sufficiently accurate forpresent purposes. Using such a model, the characteristic that defines aDC balanced waveform is that the integral of the applied voltage withtime (the applied impulse) is bounded. Note that the definition requiresthat be integral be “bounded” and not “zero.” To illustrate this point,consider a monochrome addressing waveform which uses a 300 ms×−15Vsquare pulse to drive the transition from white to black, and a 300ms×15V square pulse to drive the transition from black to white. Thiswaveform is clearly DC balanced, but the integral of applied voltage isnot zero at every point in time; this integral varies between 0 and ±4.5V-sec. However, this waveform DC is balanced in as much as the integralis bounded; the integral never reaches 9 or 18 V-sec, for example.

For further consideration of DC balanced waveforms, some definition ofterms is advisable. The term “impulse” has already been defined asmeaning the definite integral of voltage with respect to time (in V-sec)applied during a particular interval, usually an addressing pulse orpulse element. The term “impulse potential” will be used to mean the sumof all impulses applied to the display since an arbitrary starting point(typically the beginning of a series of transitions under consideration.At the starting point, the impulse potential is arbitrarily set to zero,and as impulses are applied the impulse potential rises and falls.

Using these terms, the definition of DC balance is that a waveform is DCbalanced if and only if the impulse potential is bounded. Having abounded impulse potential means that one must be able to say what theimpulse potential will be in each of a finite number of possible cases.

For a time-independent controller (i.e., a controller in which theimpulse of the waveform is influenced only by the initial and finalstates of the transition under consideration, and not dwell times,temperature, or other factors, such as the R1/R2 controller mentionedabove), in order to show that a waveform is DC balanced, it is necessaryto be able to prove that the impulse potential will be bounded aftereach transition in any infinitely long sequence of optical states. Onesufficient condition for such proof is that the impulse potential can beexpressed as a function of a fixed number of prior states, and thisprovides a working concept of DC balance for controllers forelectro-optic displays, i.e., that the impulse potential can beexpressed as a function of a finite number of prior and current opticalstates. Note that the impulse potential of any pixel of the display doesnot change from the end of one image update to the beginning of anotherimage update, because no voltage is applied during this period.

For each combination of a (finite) number of prior states, thecontroller applies a fixed impulse (the impulse determined by the datain the flash memory already mentioned), and these fixed impulses can belisted. To list them, it is necessary to enumerate prior statecombinations back by at least the number of prior states being used inthe controller (i.e. for an R1/R2 controller, the number of prior statesused in the enumeration needs to be defined for all combinations ofprior states two back).

To define the impulse potential at the end of the update, knowing thefixed impulse applied during the impulse, one needs to be able to definethe impulse potential at the beginning of the update for all states inthe enumeration. This means that the net impulse applied by a waveformmust be a function of one fewer prior state than the number needed touniquely define the impulse potential at the end. To translate this tothe problem of determining the optimum waveform to be applied by acontroller, this means that the impulse potential for a waveform must bea function of one fewer prior states than the number of states used todetermine the waveform. For example, if a controller has impulse datadetermined by three states, R1, R2, and R3 (where R3 is the gray levelimmediately prior to the initial gray level for the transition underconsideration), each combination of R1 and R2 must leave theelectro-optic medium at the same impulse potential, independent of R3.

In other words, the controller has to “know” the impulse potential ofthe electro-optic medium when it starts the transition being considered,so it can apply the right impulse to produce the proper value of impulsepotential following the transition. If the impulse potential in theabove example were allowed to vary based on all of R1, R2, and R3, then,in the next transition, there would be no way for the controller to“know” the starting impulse potential, since the R3 informationpreviously used would have been discarded.

As already indicated, the limited transitions method of the presentinvention is preferably carried out using an R1/R2 controller (i.e., acontroller in which the impulse applied during any transition dependsonly upon the initial and final gray levels of that transition), andfrom the foregoing discussion it will be seen that in such a controllerthe impulse potential must be uniquely defined as a function of R1 only.

Further complications in determining the optimum waveform arise from aphenomenon which may be called “impulse hysteresis”. Except in raresituations of extreme overdrive at the optical rails, electro-opticmedia driven with voltage of one polarity always get blacker, andelectro-optic medium driven with voltage of the opposite polarity alwaysget whiter. However, for some electro-optic media, and in particularsome encapsulated electro-optic media, the variation of optical statewith impulse displays hysteresis; as the medium is driven further towardwhite, the optical change per applied impulse unit decreases, but if thepolarity of the applied voltage is abruptly reversed so that the displayis driven in the opposed direction, the optical change per impulse unitabruptly increases. In other words the magnitude of the optical changeper impulse unit is strongly dependent not only upon the current opticalstate but also upon the direction of change of the optical state.

This impulse hysteresis produces an inherent “restoring force” tendingto bring the electro-optic medium towards middle gray levels, andconfounds efforts to drive the medium from state to state with unipolarpulses (as in general gray scale image flow) while still maintaining DCbalance. As pulses are applied, the medium rides the three-dimensionalR1/R2/impulse hysteresis surface until it reaches an equilibrium. Thisequilibrium is fixed for each pulse length and is generally in thecenter of the optical range. For example, it has been found empiricallythat driving one encapsulated four gray level electro-optic medium fromblack to dark gray required a 100 ms×−15 V unipolar impulse, but drivingit back from dark gray to black required a 300 ms×15 V unipolar impulse.This waveform was not DC balanced, for obvious reasons.

A solution to the impulse hysteresis problem is to use a bipolar drive,that is to say to drive the electro-optic medium on a (potentially)non-direct path from one gray level to the next, first applying animpulse to drive the pixel into either optical rail as required tomaintain DC balance and then applying a second impulse to reach thedesired optical state. For example, in the above situation, one could gofrom black to dark gray by applying 100 ms×−15 V of impulse, but go backfrom dark gray to white by first applying additional negative voltage,then positive voltage, riding the R1/R2 impulse curve down to the blackstate. Such indirect transitions also avoid the problem of accumulationof errors by rail stabilization of gray scale, as already discussed.

The impulse hysteresis phenomenon and the prior state dependence ofelectro-optic media, as discussed above and in the aforementionedpatents and applications, require that the waveform for each transitionvary depending upon the prior state history of the pixel beingconsidered. As described in the aforementioned 2003/0137521, the optimumwaveform for each transition may be determined (i.e., the transitiontable corresponding to the aforementioned data array may be “tuned”) byusing an initial “guessed” transition matrix to create a waveform, whichis used to address the electro-optic medium through a fixed, typicallypseudo-random or prior-state-complete series of optical states. Aprogram subtracts the actual optical state achieved in each prior statecombination from the target gray states for the same combination tocompute an error matrix, which is the same dimensions as the transitionmatrix. Each element in the error matrix corresponds to an element inthe transition matrix. If an element in the transition matrix is toohigh, the corresponding element in the error matrix will be pushedhigher. PID (proportional-integral-differential) control can then beused to drive the error matrix toward zero. There are cross-terms (eachelement in the transition matrix affects more than one element in theerror matrix) but these effects are minor and tend to decrease as themagnitudes of the values in the error matrix decrease, as the tuningproceeds through multiple iterations. (Note that sometimes the I or Dconstants of the PID controller may be set to 0, yielding PI, PD, or Pcontrol.)

When this tuning process is completed, it is found that a certain numberof prior optical states need to be in the transition matrix to achieve acertain gray level precision performance. For example, using thisprocess with one specific encapsulated electro-optic medium yielded awaveform in which the controller recorded one more prior optical statethan was in the transition matrix, and calculated the impulse in thefirst section of the waveform using arithmetic to ensure DC balance. Inthis waveform, the impulse potential was allowed to be different foreach prior state combination covered by the transition matrix.

The correlation between the number of dimensions in the transitionmatrix (“TM dimension”) and the maximum optical error for this waveformwas found to be as set out in Table 6 below:

TABLE 6 TM Dimension Maximum Optical Error (L*) 1 10.6 2 3.8 3 2.1 4 1.7

Since the limit of visual perception for the average observer is around1 L* unit, the data in this table indicate that it is very useful tohave more than one dimension in the transition matrix, with a twodimensional matrix being superior to a one dimensional, a threedimensional matrix being superior to a two dimensional, etc.

Having regard to all of the foregoing points, a preferred waveform wasdevised for the R1/R22 bit gray scale controller already mentioned. Thiswaveform maintained fixed impulse potentials for each final opticalstate R1, but used a two dimensional transition matrix. It was railstabilized, to reduce the accumulation of error, and was designed tohave low divergence during toggling because it respected the impulsehysteresis curve.

In the notation used below, numbers represent impulse. Negative impulsewas applied by applying −V (i.e. −15V) for a given time, and positiveimpulse was applied by applying +V for a given time (i.e., the waveformwas pulse width modulated), so that the magnitude of the volt-timeproduct equaled the magnitude of the impulse. Voltage modulation couldalternatively be used.

In the preferred waveform, the following sequence of impulses wasapplied during each update, reading from left to right in time:

-   -   −TM(R1,R2) IP(R1)-IP(R2) TM(R1,R2)        where “IP(Rx)” represents the relevant value from an impulse        potential matrix (in this case a vector) having one value for        each gray level, and TM(R1,R2) represents the relevant value        from a transition matrix having one value for each R1/R2        combination. TM(R1,R2) can of course be negative for certain        values of R1 and R2. (As already noted, for convenience, impulse        sequences of this type may hereinafter be abbreviated as        “−x/ΔIP/x” sequences.)

The values in the transition matrix could be adjusted as desired,without worrying about DC balance, because the net impulse of the firstand third sections of this waveform is always zero. The difference inimpulse potential between the initial and final state is applied in themiddle section of the waveform.

Empirically, it has been found that the final drive pulse almost alwayshas more effect on the final gray level than the initial pulse, so thetransition matrix for this waveform can be tuned with the same PIDapproach described above. The values set for the impulse potentialsinfluence the update speed of the waveform for fixed final gray levels.For example, all the impulse potentials could be set to zero, but thisresults in a long update time, because the final drive pulse (thirdsection) is always countered by an equally long initial pulse (firstsection). Thus, the final drive pulse, in this case, cannot be longerthan half the total update time. By careful selection of impulsepotentials, it is possible to use a much larger fraction of the totalupdate time for the final pulse; for example, one can achieve finaldrive pulses occupying more than half, and as much as 80% of the totalmaximum update time.

Preferably, the lengths of the various pulses are selected by computer,using a gradient following optimization method, like PID control, finitedifference combination evaluation, etc.

As noted in Paragraphs [0073] to [0077] of the aforementioned2003/0137521 and above, transitions in electro-optic media are typicallytemperature sensitive, and it has been found that the uncompensatedstability of gray levels versus temperature is increased when all of thetransitions to a particular gray level always come from the same opticalrail. The reason for this is straightforward; as the temperature varies,the switching speed of the electro-optic medium becomes gets faster orslower. Suppose that, in a 2 bit gray level display, the dark gray tolight gray transition bounces off the black rail, but the white to lightgray transition bounces off the white rail. If the switching speed ofthe medium becomes slower, the light gray state addressed from blackwill become darker, but the light gray state addressed from black willbecome lighter. Thus, it is important for a temperature stable waveformthat a given gray level always be approached from the same “side”, i.e.,that the final pulse of the waveform always be of the same polarity. Inthe preferred drive scheme described above using the

-   -   −TM(R1,R2) IP(R1)-IP(R2) TM(R1,R2)        sequence, this requires choosing the TM(R1,R2) values so that        the sign of each value is dependent only on R1, at least for        some gray levels. One preferred approach is to allow the TM        values to be of either sign for the black and white states, but        positive only for light gray, and negative only for dark gray,        and thus that the intermediate gray levels be approached only        from the nearer optical rail.

This preferred waveform is fully compatible with techniques such asinsertion of short pause periods into the waveform to increase impulseresolution, as described below.

As already indicated, the aforementioned −x/ΔIP/x pulse sequences may bemodified to contain additional pulses. One such modification allows theinclusion of an additional class of pulses, hereinafter referred to as“y” pulses. “y” pulses are characterized by being of the form [+y][−y],where y is an impulse value, and may be either negative or positive (inother words, the form [−y][+y] is equally valid. The y pulse is distinctfrom the previously-described “x” pulses, in that the [−x] and [+x]halves of the “x” pulse pair are disposed before and after the ΔIPpulse, whereas the “y” pulses can be disposed at other locations withinthe pulse sequence.

A second such modification adds a 0 V “pulse” (i.e., a period when novoltage is applied to the relevant pixel) at an arbitrary point withinthe pulse sequence to improve the performance of that sequence, by, forexample, shifting the gray level resulting from the transition up ordown by a small amount, or reducing or changing the impact of priorstate information on the final state of the pixel. Such 0 V sections maybe inserted either between the different pulse elements, or in themiddle of a single pulse element.

A preferred method for constructing a rail-stabilized waveform, using atransition table as described in the aforementioned 2003/0137521 is asfollows:

(a) set the value (typically derived empirically) of the impulsepotential for each gray level, and insert into the transition table theappropriate ΔIP pulse for each transition;

(b) for each transition, pick a value for x, and insert a −x pulsebefore, and a +x pulse after, the ΔIP pulse (as already noted, the valueof x may be negative, so the −x and +x pulses can have either polarity);

(c) for each transition, pick a value for y, and insert a −y and +ypulses into the pulse sequence. The −y/+y pulse combination may beinserted into the sequence at any pulse boundary, for example before the−x pulse, before the ΔIP pulse, before the +x pulse, or after the +xpulse;

(d) for each transition, insert n frames, where n=0 or more, of 0 V atany point or points in the pulse sequence; and

(e) repeat the above steps as many times as desired, until the waveformperformance reaches the desired level.

This process will be illustrated with reference to the accompanyingdrawings. FIG. 12 shows the basic −x/ΔIP/+x structure of the waveformfor one transition, it being assumed for the sake of illustration thatthe values of both x and ΔIP are positive. Unless it is desired toprovide a 0 V interval between the ΔIP and the +x pulses, it is notnecessary to reduce the voltage applied at the junction between thesetwo pulses, so that the ΔIP and +x pulses form, in effect, one longpositive pulse.

FIG. 13 illustrates symbolically the insertion of a [−y][+y] pair ofpulses into the basic −x/ΔIP/+x waveform shown in FIG. 12. The −y and +ypulses do not have to be consecutive, but can be inserted at differentplaces into the original waveform. There are two especially advantageousspecial cases.

In the first special case, the “−y, +y” pulse pair is placed at thebeginning of the −x/ΔIP/+x waveform, before the −x pulse, to produce thewaveform shown in FIG. 14. It has been found that, when y and x are ofopposite sign, as illustrated in FIG. 14, the final optical state can befinely tuned by even moderately coarse adjustment of the duration y.Thus, the value of x can be adjusted for coarse control and the value ofy for final control of the final optical state of the electro-opticmedium. This is believed to happen because the y pulse augments the −xpulse, thus changing the degree to which the electro-optic medium ispushed into one of its optical rails. The degree of pushing into one ofthe optical rails is known to give fine adjustment of the final opticalstate after a pulse away from that optical rail (in this case, providedby the x pulse).

In a second special case, illustrated in FIG. 15, the −y pulse is againplaced at the beginning of the −x/ΔIP/+x waveform, before the −x pulse,but the +y pulse is placed at the end of the waveform, after the +xpulse. In this type of waveform, the final y pulse provides coarsetuning because the final optical state is very sensitive to themagnitude of y. The x pulse provides a finer tuning, since the finaloptical state typically does not depend as strongly on the magnitude ofthe drive into the optical rail.

As already indicated, more than one pair of “y” pulses may be insertedinto the basic −x/ΔIP/+x waveform to allow “fine tuning” of gray scalelevels of the electro-optic medium, and the impulses of such multiplepairs of “y” pulses may differ from one another. FIG. 16 illustratessymbolically, in a manner similar to that of FIG. 13, the insertion of asecond pair of y-type pulses (denoted “−z”, “+z”) into the waveform ofFIG. 15. It will readily be apparent that since the −z and +z pulses canbe introduced at any pulse boundary of the waveform shown in FIG. 15, alarge number of different waveforms can result from the introduction ofthe −z and +z pulses. A preferred resulting waveform is shown in FIG.17; this type of waveform is useful for fine tuning of the final opticalstate, for the following reasons. Consider the situation without the −zand +z pulses (i.e. the FIG. 15 waveform discussed above). The x pulseelement is used for fine tuning, and the final optical state can bedecreased by increasing x and increased by decreasing x. However, it isundesirable to decrease x beyond a certain point because then theelectro-optic medium is not brought sufficiently close to an opticalrail, as required for stability of the waveform. To avoid this problem,instead of decreasing x, one can (in effect) increase the −x pulsewithout changing the +x pulse by adding the −z, +z pulse pair as shownin FIG. 17, with z having the opposite sign from x. The +z pulseaugments the −x pulse, while the −z pulse maintains the transition atthe desired net impulse, thus maintaining an overall DC balancedtransition table.

In the limited transitions waveform scheme of the present invention, itis acceptable for the “diagonal elements” (the transition table elementscorresponding to null transitions in which the initial and final graylevels are the same, so called because in a normal matrix representationof a transition table such elements lie on the leading diagonal; suchdiagonal elements have ΔIP=0) to contain both x and y pulses. Any giventransition table element may contain zero or more sets of x and/or ypulses.

The limited transitions method of the present invention may also makeuse of pause periods between adjacent frames of a transition; such pauseperiods are discussed in more detail below with reference to theinterrupted scanning method of the present invention. Typically, in anactive matrix display, the pixels are divided into a series of groups(normally a plurality of rows), each of these plurality of groups isselected in succession (i.e., typically the rows of the matrix arescanned) and there is applied to each of the pixels in the selectedgroup either a drive voltage or a non-drive voltage. The scanning of allthe groups of pixels is completed within a frame period. The scanning ofthe groups of pixels is repeated, and, in a typical electro-opticdisplay, the scanning will be repeated more than once during the groupof frames (conveniently referred to as a superframe) required for acomplete rewriting of the display. Normally, a fixed scan rate is usedfor updating, for example 50 Hz, which allows for 20 msec frames.However, this frame length may provide insufficient resolution foroptimal waveform performance. In many cases, frames of length t/2 aredesirable, for example 10 msec frames in a normally 20 msec frame lengthwaveform. It is possible to combine frames of differing delay times togenerate a pulse resolution of n/2. To take one specific case a singleframe of length 1.5*t may be inserted at the beginning of the waveform,and a similar frame at the end of the waveform (immediately before theterminating 0 V frame, which should occur at the ordinary frame rate andwhich is normally used at the end of the waveform to prevent undesirableeffects caused by varying residual voltages on pixels). The two longerframes can be realized by simply adding a 0.5*t delay time between thescanning of two adjacent frames. The waveform would then have thestructure:

t ms frame:t/2 ms delay:t ms frame [ . . . ] t ms frame:t/2 ms delay:tms frame (all outputs to 0V)

For a normal frame length of 20 msec, the initial and final frames plustheir respective delays would amount to 30 msec each.

Using this waveform, structure, the initial and final pulses are allowedto vary by 10 msec in length, by using the following algorithm:

(a) If the length of the initial pulse is evenly divisible by t, thenthe first frame consists of a 0 V drive, and a corresponding number offrames of t ms are activated to achieve the desired pulse length; or

(b) If the length of the initial pulse leaves a remainder of t/2 whendivided by t, then the first frame of 1.5*t is active, and acorresponding number of t msec frames following the initial frame areactivated to achieve the desired pulse length.

The same algorithm is followed for the final pulse. Note that theinitial and final pulses must be start- and end-justified, respectively,for this algorithm to work properly. In addition, in order to maintainDC balance, the initial and final pulses may be corresponding parts ofα−x/+x pair.

Whether or not pause periods are employed, it has been found that theeffect of the waveform used to effect a transition is modified by thepresence of a period of zero voltage (in effect a time delay) during orbefore any of the pulses in the waveform, and the limited transitionsmethod of the present invention may include periods of zero voltagewithin or between successive pulses in the waveform, i.e., the waveformmay be “non-contiguous” as that term is used above and in theaforementioned application Ser. No. 10/814,205. FIGS. 18 to 20illustrate variations of the basic −x/ΔIP/+x waveform of FIG. 12incorporating such zero voltage periods. In the waveform of FIG. 18, atime delay is inserted between the −x pulse and the ΔIP pulse. In thewaveform of FIG. 19, a time delay is inserted within the ΔIP pulse, or,which amounts to the same thing, the ΔIP pulse is split into twoseparate pulses separated by the time delay. The waveform of FIG. 20 issimilar to that of FIG. 19, except that the time delay is insertedwithin the +x pulse. Time delays can be incorporated into a waveform toachieve optical states not achievable without such delays. Time delayscan also be used to fine-tune the final optical state. This fine-tuningability is important, because in an active matrix drive, the timeresolution of each pulse is defined by the scan rate of the display. Thetime resolution offered by the scan rate can be coarse enough thatprecise final optical states cannot be achieved without some additionalmeans of fine tuning.

Interrupted Scanning Method of the Present Invention

As already mentioned, this invention provides an “interrupted scanning”method for driving an electro-optic display having a plurality of pixelsdivided into a plurality of groups. The method comprises selecting eachof the plurality of groups of pixels in succession and applying to eachof the pixels in the selected group either a drive voltage or anon-drive voltage, the scanning of all the groups of pixels beingcompleted in a first frame period. The scanning of the groups of pixelsis repeated during a second frame period (it being understood that anyspecific pixel may have the drive voltage applied during the first frameperiod and the non-drive voltage applied during the second frame period,or vice versa). In the interrupted scanning method invention, thescanning of the groups of pixels is interrupted during a pause periodbetween the first and second frame periods, this pause period being notlonger than the first or second frame period. In this method, the firstand second frame periods are typically equal in length, and the lengthof the pause period is typically a sub-multiple (desirably, one half,one fourth etc.) of the length of one of the frame periods.

The interrupted scanning method may include multiple pause periodsbetween different pairs of adjacent frame periods. Such multiple pauseperiods are preferably of substantially equal length, and the totallength the multiple pause periods is preferably equal to either onecomplete frame period, or equal to one frame period less one pauseperiod. For example, as discussed in more detail below, one embodimentof the first method might use multiple 20 ms frame periods, and eitherthree or four 5 ms pause periods.

In this interrupted scanning method, the groups of pixels will of coursetypically be the rows of a conventional row/column active matrix pixelarray. The interrupted scanning method comprises selecting each of theplurality of groups of pixels in succession (i.e., typically, scanningthe rows of the matrix) and applying to each of the pixels in theselected group either a drive voltage or a non-drive voltage, thescanning of all the groups of pixels being completed in a first frameperiod. The scanning of the groups of pixels is repeated, and in atypical electro-optic display, the scanning will be repeated more thanonce during the superframe required for a complete rewriting of thedisplay. The scanning of the groups of pixels is interrupted during apause period between the first and second frame periods, this pauseperiod being not longer than the first or second frame period.

Although a drive voltage is only applied to any specific pixel electrodefor one line address time during each scan, the drive voltage persistson the pixel electrodes during the time between successive selections ofthe same line, only slowly decaying, so that the pixel continues todriven during the time when other lines of the matrix are beingselected, and the interrupted scanning method relies upon this continueddriving of the pixel during its “non-selected” time. Ignoring for themoment the slow decay of the voltage on the pixel electrode during itsnon-selected time, a pixel which is set to the driving voltage duringthe frame period immediately preceding the pause period will continue toexperience the driving voltage during the pause period, so that for sucha pixel the preceding frame period is in effect lengthened by the lengthof the pause period. On the other hand, a pixel which is set to thenon-driving (typically zero) voltage during the frame period immediatelypreceding the pause period will continue to experience the zero voltageduring the pause period. It may be desirable to adjust the length of thepause period to allow for the slow decay of the voltage on the pixelelectrode in order to ensure that the total impulse delivered to thepixel during the pause period has the desired value.

To take a simple example of the interrupted scanning method for purposesof illustration, consider a simple pulse width modulated drive schemehaving a superframe consisting of a plurality of (say 10) 20 ms frames.Typically, the last frame of the superframe will set all pixels to thenon-driving voltage, since bistable electro-optic displays are normallyonly driven when the displayed image is to be changed, or at relativelylong intervals when it is deemed desirable to refresh the displayedimage, so that each superframe will typically be followed by a lengthyperiod in which the display is not driven, and it is highly desirable toset all pixels to the non-driving voltage at the end of the superframein order to prevent rapid changes in some pixels during this lengthynon-driven period. To modify such a drive scheme in accordance with theinterrupted scanning method of the present invention, a 10 ms pauseperiod may be inserted between two successive 20 ms frames, and thissimple modification halves the maximum possible difference between theapplied impulse and the impulse ideally needed to complete a giventransition, thereby in practice approximately halving the maximumdeviation in achieved gray scale level. The 10 ms pause period isconveniently inserted after the penultimate frame in each superframe butmay be inserted at other points in the superframe if desired.

In practice, it is desirable, in this example, not only to insert the 10ms pause period but also to insert one additional 20 ms frame into eachsuperframe. The unmodified drive scheme enables one to apply to anygiven pixel impulses of:

-   -   0, 20, 40, 60 . . . 160, 180 units        where one impulse unit is defined as the impulse resulting from        application of the driving voltage for 1 ms. Thus, the maximum        difference between the available impulses and the ideal impulse        for a given transition is 10 units. (Since the last frame of the        superframe sets all pixels to the non-driving voltage, only the        first nine frames of the superframe are available for        application of the driving voltage.) As already explained, any        pixel which is set to the driving voltage in the frame preceding        the pause period continues to experience this driving voltage        for a period equal to the frame period plus the pause period,        and thus experiences an impulse of 30 units instead of 20 units        for this frame. Accordingly, the modified drive scheme permits        one to apply to any given pixel impulses of:    -   0, 20, 30, 40, 50, 60 units etc.

Insertion of the additional frame into the superframe is desirable toenable the modified drive scheme to deliver an impulse of exactly 180units. Since any impulse which is an exact multiple of 20 units requiresthat the relevant pixel be set to the non-driving voltage during theframe preceding the pause period, achieving an impulse of exactly 180units requires an 11-frame superframe, so that any pixel to receive the180 impulse can be set to the driving voltage during 9 frames, to thenon-driving voltage during the frame preceding the pause period, and (asalways) to the non-driving voltage during the last frame of thesuperframe. Thus, when using the modified drive scheme, the maximumdifference between the available impulses and the ideal impulse for agiven transition is reduced to 5 units. (Although the modified drivescheme is not capable of applying an impulse of 10 units, in practicethis is of little consequence. To produce reasonably consistent grayscale levels, the number of available impulse levels has to besubstantially larger than the number of gray levels of the display, sothat it is unlikely that any gray scale transition will require animpulse as small as 10 units.)

The pause periods can of course be of any number and length required toachieve the desired control over the impulse applied. For example,instead of modifying the aforementioned drive scheme to include one 10ms pause period, the drive scheme could be modified to include three 5ms pause periods after different 20 ms drive frames, desirably with theaddition to the drive scheme of three further 20 ms drive frames notfollowed by pause periods. This modified drive scheme permits one toapply to any given pixel impulses of:

-   -   0, 20, 25, 30, 35 . . . 170, 175, 180 units        thereby reducing the maximum difference between the available        impulses and the ideal impulse for a given transition is reduced        to 2.5 units, a four-fold reduction as compared with the        original unmodified drive scheme.

The preceding discussion of the interrupted scanning method has ignoredthe question of polarity of the applied impulses. As discussed above andin the aforementioned 2003/0137521, bistable electro-optic media requireapplication of impulses of both polarities. In some drive schemes, suchas slide show drive schemes (cf. the discussion of FIGS. 9 and 10above), before a new image is written to the display, all the pixels ofthe display are first driven to one extreme optical state, either blackor white, and thereafter the pixels are driven to their final graystates by impulses of a single polarity. Such drive schemes can bemodified in accordance with the interrupted scanning method in themanner already described. Other drive schemes require application ofimpulses of both polarities to drive the pixels to their final graystates. The impulses of the two polarities may be applied in separateframes (see, for example, Paragraphs [0128] to [0132] of theaforementioned 2003/0137521 and the discussion of Table 3 above) or, asdiscussed above, impulses of the two polarities may be applied in thesame frames, for example using a tri-level drive scheme in which thecommon front electrode is held at a voltage of V/2, while individualpixel electrodes are held at 0, V/2 or V. When the impulses of the twopolarities are applied in separate frames, the interrupted scanningmethod is desirably effected by providing at least two separate pauseperiods, one following a frame in which impulses of one polarity areapplied and the second following a frame in which impulses of theopposed polarity are applied. However, when using a drive scheme inwhich impulses of both polarities are applied in the same frames, theinterrupted scanning method may make use of only a single pause periodsince, as will be apparent from the foregoing discussion, the effect ofincluding a pause period after a frame is to increase the magnitude ofthe impulse applied to any pixel to which a driving voltage was appliedin the frame, regardless of the polarity of this driving voltage.

Also as discussed in the aforementioned 2003/0137521 and above, manybistable electro-optic media are desirably driven with drive schemeswhich achieve long term direct current (DC) balance, and such DC balanceis conveniently effected using a drive scheme in which a DC balancesection, which does not substantially change the gray level of thepixel, is applied before the main drive section, which does change thegray level, the two sections being chosen so that the algebraic sum ofthe impulses applied is zero or at least very small. If the main drivesection is modified in accordance with the interrupted scanning method,it is highly desirable that the DC balance section be modified toprevent the additional impulses caused by the insertion of the pauseperiods accumulating to cause substantial DC imbalance. However, it isnot necessary that the DC balance section be modified in a manner whichis an exact mirror image of the modification of the main drive section,since the DC balance section can have gaps (zero voltage frames) andmost electro-optic medium are not harmed by short term DC imbalances.Thus, in the drive scheme discussed above using a single 10 ms pauseperiod inserted among ten 20 ms frames, DC balance can be achieved bymaking the first frame of the drive scheme 30 ms in duration. Applyingor not applying a driving voltage to a pixel during this frame bringsthe overall impulse to a multiple of 20 units, so that this impulse canreadily be balanced later. In the drive scheme using three 5 ms pauseperiods, the first two frames of the drive scheme can similarly be 25and 30 ms in duration (in either order), again bringing the overallimpulse to a multiple of 20 units.

From the foregoing description, it will be seen that the interruptedscanning method of the present invention requires a trade-off betweenincreased addressing time caused by the need to include one additionalframe in each superframe for each pause period inserted, and theimproved control of impulse and hence gray scale produced by the method.However, the interrupted scanning method can provide very substantialimprovement in impulse control with only modest increase in addressingtime; for example, the drive scheme described above in which asuperframe comprising ten 20 ms frames is modified to include three 5 mspause periods yields a four-fold improvement in impulse accuracy at thecost of less than a 40 percent increase in addressing time.

Balanced Gray Level Method of the Present Invention

As already mentioned, this invention also provides a balanced gray levelmethod for driving an electro-optic display having a plurality of pixelsarranged in an array. The pixels are driven with a pulse width modulatedwaveform capable of applying a plurality of differing impulses. Drivecircuitry stores data indicating whether application of a given impulsewill produce a gray level higher or lower than a desired gray level.When two adjacent pixels are both required to be in the same gray level,the impulses applied to the two pixels are adjusted to that one pixel isbelow the desired gray level, while the other pixel is above the desiredgray level.

In a preferred form of this method, the pixels are divided into twogroups, hereinafter designated “even” and “odd”. The two groups ofpixels may be arranged in a checkerboard pattern (so that the pixels ineach row and column alternate between the two groups) or in otherarrangements as described above and in the aforementioned 2003/0137521,Paragraphs [0181] to [0183] and [0199] to [0202], provided that eachpixel has at least one neighbor of the opposite group, and differentdrive schemes are used for the two groups. If the stored data indicatesthat one of the available impulses will produce substantially thedesired gray level transition, this impulse is applied for thattransition for both the even and odd pixels. However, if the stored dataindicates that the impulse required for a particular gray leveltransition is substantially half-way between two of the availableimpulses, one of these impulses is used for the transition in evenpixels and the other of these impulses is used for the transition in oddpixels. Thus, if two adjacent pixels are intended to be in the same graystate (the condition where precise control of gray scale is of maximumimportance) one of these pixels will have a gray level slightly abovethe desired level, while the other will have a gray level slightly belowthe desired level. Ocular and optical averaging will result in anaverage of the two gray levels being seen, thus producing an apparentgray level closer to the desired level than can be achieved with theavailable impulses. In effect, this balanced gray level method usessmall-signal spatial dithering (applied to correct errors in appliedimpulse) superimposed on large signal true gray scale to increase by afactor of two the available impulse levels. Since each pixel is still atapproximately the correct gray scale level, the effective resolution ofthe display is not compromised.

A complete implementation of the necessary calculations, in MATHLABpseudo-code is given below. The floor function rounds down to thenearest integer, and the mod function computes the remainder of itsfirst argument divided by its second argument:

quotient= floor(desired_impuslse) remainder = mod(desired_impulse,1) ifremainder <= 0.25 even_parity_impulse = quotient odd_parity_impuslse =quotient else if remainder <= 0.75 even_parity_impulse = quotient + 1odd_parity_impulse = quotient else even_parity_impulse = quotient + 1odd_parity_impulse = quotient + 1 end.

In some drive schemes previously described, for example the cyclic RSGSdrive scheme described above with reference to FIGS. 11A and 11B, thepixels of the display are already divided into two groups and differentdrive schemes are applied to the two groups, so that the magnitude ofthe impulses needed to achieve the desired gray level will be differentof the two groups. Such “two group” drive schemes can be modified inaccordance with the balanced gray level method but the detailedimplementation of the method differs somewhat from the simple casediscussed above. Instead of simply comparing the available impulses withthat required for the desired transition, one calculates the errors ingray scale for the two groups separately, takes the arithmetic averageof the errors, and determines whether this arithmetic average would bereduced by shifting one of the groups to a different available impulse.Note that in this case, the reduction in arithmetic average may differdepending upon which group is shifted to a different impulse, andobviously whichever shift produces the smaller average should beeffected.

Again, this method can be thought of as small-signal spatial ditheringimplemented on top of large signal intrinsic gray scale, with the smallsignal dithering used to correct for errors in impulse due to thelimitation of the pulse width modulation drive scheme used. Because eachpixel is still approximately at the correct gray level in this scheme,and the corrections are only to correct for impulse rounding errors,effective display resolution is not compromised. To put it another way,this method implements small signal spatial dithering on top of largesignal true gray scale.

The various methods of the present invention may make use of variousadditional variations and techniques described in the aforementionedapplications, especially the aforementioned 2003/0137521 and applicationSer. No. 10/814,205, which variations and techniques are described inthe “Additional Background Information” section below. It will beappreciated that in the overall waveform used to drive an electro-opticdisplay, in at least some cases certain transitions may be effected inaccordance with the various methods of the present invention, whileother transitions may not make use of the methods of the presentinvention but may make use of other types of transitions describedbelow.

Additional Background Information

Part A: Non-Contiguous Addressing

As already briefly indicated, the present methods may make use of“non-contiguous addressing” as that term in used in the aforementionedapplication Ser. No. 10/814,205. As there described, such non-contiguousaddressing has two principal variants, a DC imbalanced variant and a DCbalanced variant. The DC imbalanced variant effects at least onetransition between gray levels using an output signal which has anon-zero net impulse (i.e., the length of positive and negative segmentsis not equal), and therefore is not internally DC balanced, and isnon-contiguous, (i.e. the pulse contains portions of zero voltage oropposite polarity). The output signal used in the non-contiguousaddressing method may or may not be non-periodic (i.e., it may or maynot consist of repeating units such as +/−/+/− or ++/−−/++/−−).

Such a non-contiguous waveform (which may hereinafter be referred to asa “fine tuning” or “FT” waveform) may have no frames of oppositepolarity, and/or may include only three voltage levels, +V, 0, and −Vwith respect to the effective front plane voltage of the display(assuming, as is typically the case, an active matrix display having apixel electrode associated with each pixel and a common front electrodeextending across multiple pixels, and typically the whole display, sothe electric field applied to any pixel of the electro-optic medium isdetermined by the voltage difference between its associated pixelelectrode and the common front electrode). Alternatively, an FT waveformmay include more than three voltage levels. An FT waveform may consistof any one of the types of waveforms described above (such n-prepulseetc), with a non-contiguous waveform appended.

An FT waveform may (and typically will) be dependent on one or moreprior image states, and can be used in order to achieve a smaller changein optical state than can be achieved using standard pulse widthmodulation (PWM) techniques. (Thus, the exact FT waveform employed willvary from one transition to another in a look-up table, in contrast tocertain prior art waveforms in which pulses of alternating polarity areemployed, for example, allegedly to prevent sticking of electrophoreticparticles to surfaces such as capsule walls.) In a preferred variant ofthe non-contiguous addressing method, there is provided a combination ofall waveforms required to achieve all allowed optical transitions in adisplay (a “transition matrix”), in which at least one waveform is an FTwaveform of the present invention and the combination of waveforms isDC-balanced. In another preferred variant of the non-contiguousaddressing method, the lengths of all voltage segments are integermultiples of a single interval (the “frame time”); a voltage segment isa portion of a waveform in which the voltage remains constant.

Non-contiguous addressing is based upon the discovery that, in manyimpulse driven electro-optic media, a waveform which has zero netimpulse, and which thus might theoretically be expected to effect nooverall change in the gray level of a pixel, can in fact, because ofcertain non-linear effects in the properties of such media, effect asmall change in gray level, which can be used to achieve fineradjustment of gray levels than is possible using a simple PWM drivescheme or drivers with limited ability to vary the width and/or heightof a pulse. The pulses which may up such a “fine tuning” waveform may beseparate from the “major drive” pulses which effect a major change ingray level, and may precede or follow such major drive pulses.Alternatively, in some cases, the fine adjustment pulses may beintermingled with the major drive pulses, either a separate block offine tuning pulses at a single point in the sequence of major drivepulses, or interspersed singly or in small groups at multiple points inthe sequence of major drive pulses.

Although non-contiguous addressing has very general applicability, itwill primarily be described using as an example drive schemes usingsource drivers with three voltage outputs (positive, negative, and zero)and waveforms constructed from the following three types of waveformelements (since it is believed that the necessary modifications of thepresent invention for use with other types of drivers and waveformelements will readily be apparent to those skilled in the technology ofelectro-optic displays):

1) Saturation pulse: A sequence of frames with voltages of one sign orone sign and zero volts that drives the reflectance approximately to oneextreme optical state (an optical rail, either the darkest state, herecalled the black state, or the brightest state, here called the whitestate);

2) Set pulse: A sequence of frames with voltages of one sign or one signand zero volts that drives the reflectance approximately to a desiredgray level (black, white or an intermediate gray level); and

3) FT sequence: A sequence of frames with voltages that are individuallyselected to be positive, negative, or zero, such that the optical stateof the ink is moved much less than a single-signed sequence of the samelength. Examples of FT drive sequences having a total length of fivescan frames are: [+ − + − −] (here, the voltage of each frame isrepresented sequentially by a + for positive voltage, 0 for zerovoltage, and − for a negative voltage), [− − 0 + +], [0 0 0 0 0], [0 0 +− 0], and [0 − + 0 0]. These sequences are shown schematically in FIGS.21A-21E respectively of the accompanying drawings, in which the circlesrepresent the starting and end points of the FT sequence, and there arefive scan frames between these points.

An FT sequence may be used either to allow fine control of the opticalstate, as previously described, or to produce a change in the opticalstate similar to that for a sequence of monopolar (single-signed)voltages but having a different net voltage impulse (where impulse isdefined as the integral of the applied voltage over time). FT sequencesin the waveform can thus be used as a tool to achieve DC balance.

The use of an FT sequence to achieve fine control of the optical statewill first be described. In FIG. 22, the optical states achievable usingzero, one, two, three, or more frames of a monopolar voltage areindicated schematically as points on the reflectivity axis. From thisFigure, it will be seen that the length of the monopolar pulse can bechosen to achieve a reflectance represented by its corresponding pointon this axis. However, one may wish to achieve a gray level, such asthat indicated by “target” in FIG. 22, that is not well approximated byany of these gray levels. An FT sequence can be used to fine-tune thereflectance to the desired state, either by fine tuning the final stateachieved after a monopolar drive pulse, or by fine-tuning the initialstate and then using a monopolar drive sequence.

A first example of an FT sequence, shown in FIG. 23, shows an FTsequence being used after a two-pulse monopolar drive. The FT sequenceis used to fine-tune the final optical state to the target state. LikeFIG. 22, FIG. 23 shows the optical states achievable using variousnumbers of scan frames, as indicated by the solid points. The targetoptical state is also shown. The optical change by applying two scanframes is indicated, as is an optical shift induced by the FT sequence.

A second example of an FT sequence is shown in FIG. 24; in this case,the FT sequence is used first to fine tune the optical state into aposition where a monopolar drive sequence can be used to achieve thedesired optical state. The optical states achievable after the FTsequence are shown by the open circles in FIG. 24.

An FT sequence can also be used with a limited transitions waveform ofthe present invention, such as a rail-stabilized gray scale waveform,such as that described above with reference to FIGS. 11A and 11B. Asmentioned above, the essence of a limited transitions waveform is that agiven pixel is only allowed to make a limited number of gray-to-graytransitions before being driven to one of its extreme optical states.Thus, such waveforms use frequent drives into the extreme optical states(referred to as optical rails) to reduce optical errors whilemaintaining DC balance (where DC balance is a net voltage impulse ofzero and is described in more detail below). Well resolved gray scalecan be achieved using these waveforms by selecting fine-adjust voltagesfor one or more scan frames. However, if these fine-adjust voltages arenot available, another method must be used to achieve fine tuning,preferably while maintaining DC balance as well. FT sequences may beused to achieve these goals.

First, consider a cyclic version of a rail-stabilized grayscalewaveform, in which each transition consists of zero, one, or twosaturation pulses (pulses which drive the pixel into an optical rail)followed by a set pulse as described above (which takes the pixel to thedesired gray level). To illustrate how FT sequences can be used in thiswaveform, a symbolic notation will be used for the waveform elements:“sat” to represent a saturation pulse; “set” to represent a set pulse;and “N” to represent an FT drive sequence. The three basic types ofcyclic rail-stabilized grayscale waveforms are:

set (for example, transition 1104 in FIG. 11A)

sat-set (for example, transition 1106/1108 in FIG. 11A)

sat-sat′-set (for example, transition 1116/1118/1120 in FIG. 11A)

where sat and sat′ are two distinct saturation pulses.

Modification of the first of these types with an FT sequence gives thefollowing waveforms:

N-set

set-N

that is, an FT sequence followed by a set pulse or the same elements inreverse order.

Modification of the second of these types with one or more FT sequencesgives, for example, the following FT-modified waveforms:

N-sat-set

sat-N-set

sat-set-N

sat-N-set-N′

N-sat-set-N′

N-sat-N′-set

N-sat-N′-set-N″

where N,N′, and N″ are three FT sequences, which may or may not bedifferent from one another.

Modification of the second of these types can be achieved byinterspersing FT sequences between the three waveform elements followingessentially the previously described forms. An incomplete list ofexamples includes:

N-sat-sat′-set

N-sat-sat′-set-N′

sat-N-sat′-N′-set-N″

N-sat-N′-sat′-N″-set-N′″.

Another base waveform which can be modified with an FT sequence is thesingle-pulse slide show gray scale with drive to black (or white). Inthis waveform, the optical state is first brought to an optical rail,then to the desired image. The waveform of each transition can besymbolically represented by either of the two sequences:

sat-set

set.

Such a waveform may be modified by inclusion of FT drive sequenceelements in essentially the same manner as already described for therail-stabilized gray scale sequence, to produce sequences such as:

sat-set-N

sat-N-set

etc.

The above two examples describe the insertion of FT sequences before orafter saturation and set pulse elements of a waveform. It may beadvantageous to insert FT sequences part way through a saturation or setpulse, that is the base sequence:

sat-set

would be modified to a form such as:

{sat, part I}-N-{sat, part II}-set

or

sat-{set, part I}-N-{set, part II}

As already indicated, it has been discovered that the optical state ofmany electro-optic media achieved after a series of transitions issensitive to the prior optical states and also to the time spent inthose prior optical states, and methods have been described forcompensating for prior state and prior dwell time sensitivities byadjusting the transition waveform accordingly. FT sequences can be usedin a similar manner to compensate for prior optical states and/or priordwell times.

To describe this concept in more detail, consider a sequence of graylevels that are to be represented on a particular pixel; these levelsare denoted R₁, R₂, R₃, R₄, and so on, where R₁ denotes the next desired(final) gray level of the transition being considered, R₂ is the initialgray level for that transition, R₃ is the first prior gray level, R₄ isthe second prior gray level and so on. The gray level sequence can thenbe represented by:

-   -   R_(n) R_(n-1) R_(n-2) . . . R₃ R₂ R₁        The dwell time prior to gray level i is denoted D_(i). D_(i) may        represent the number of frame scans of dwell in gray level i.

The FT sequences described above could be chosen to be appropriate forthe transition from the current to the desired gray level. In thesimplest form, these FT sequences are then functions of the current anddesired gray level, as represented symbolically by:

-   -   N=N(R₂, R₁)        to indicate that the FT sequence N depends upon R₂ and R₁.

To improve device performance, and specifically to reduce residual graylevel shifts correlated to prior images, it is advantageous to makesmall adjustments to a transition waveform. Selection of FT sequencescould be used to achieve these adjustments. Various FT sequences giverise to various final optical states. A different FT sequence may bechosen for different optical histories of a given pixel. For example, tocompensate for the first prior image (R₃), one could choose an FTsequence that depends on R₃, as represented by:

-   -   N=N(R₃, R₂, R₁)        That is, an FT sequence could be selected based not only on R₁        and R₂, but also on R₃.

Generalizing this concept, the FT sequence can be made dependent on anarbitrary number of prior gray levels and/or on an arbitrary number ofprior dwell times, as represented symbolically by:

-   -   N=N(D_(m), D_(m-1), . . . D₃, D₂; R_(n), R_(n-1), . . . R₃, R₂,        R₁)        where the symbol D_(k) represents the dwell time spent in the        gray level R_(k) and the number of optical states, n, need not        equal the number of dwell times, m, required in the FT        determination function. Thus FT sequences may be functions of        prior images and/or prior and current gray level dwell times.

As a special case of this general concept, it has been found thatinsertion of zero voltage scan frames into an otherwise monopolar pulsecan change the final optical state achieved. For example, the opticalstate achieved after the sequence of FIG. 25, into which a zero voltagescan frame has been inserted, will differ somewhat from the opticalstate achieved after the corresponding monopolar sequence of FIG. 26,with no zero voltage scan frame but the same total impulse as thesequence of FIG. 25.

It has also been found that the impact of a given pulse on the finaloptical state depends upon the length of delay between this pulse and aprevious pulse. Thus, one can insert zero voltage frames between pulseelements to achieve fine tuning of a waveform.

The present methods may extend to the use of FT drive elements andinsertion of zero-volt scan frames in monopolar drive elements in otherwaveform structures. Other examples include but are not limited todouble-prepulse (including triple-prepulse, quadruple-prepulse and soon) slide show gray scale waveforms, where both optical rails arevisited (more than once in the case of higher numbers of prepulses) ingoing from one optical state to another, and other forms ofrail-stabilized gray scale waveforms. FT sequences could also be used ingeneral image flow gray scale waveforms, where direct transitions aremade between gray level.

While insertion of zero voltage frames can be thought of as a specificexample of insertion of an FT sequence, where the FT sequence is allzeros, attention is directed to this special case because it has beenfound to be effective in modifying final optical states.

The preceding discussion has focused on the use of FT sequences toachieve fine tuning of gray levels. The use of such FT sequences toachieve DC balance will now be considered. FT sequences can be used tochange the degree of DC imbalance (preferably to reduce or eliminate DCimbalance) in a waveform. By DC balance is meant that all full-circuitgray level sequences (sequences that begin and end with the same graylevel), have zero net voltage impulse. A waveform can be made DCbalanced or less strongly DC imbalanced by use of one or more FTsequences, taking advantage of the fact that FT sequences can either (a)change the optical state in the same way as a saturation or set pulsebut with a substantially different net voltage impulse; or (b) result inan insubstantial change in the optical state but with a net DCimbalance.

The following illustration shows how FT sequences can be used to achieveDC balance. In this example, a set pulse can be of variable length,namely one, two, three or more scan frames. The final gray levelsachieved for each of the number of scan frames are shown in FIG. 27, inwhich the number next to each point represents the number of scan framesused to achieve the gray level.

FIG. 27 shows the optical states available using scan frames of positivevoltage, monopolar drive where the number labels specify the number ofmonopolar frames used to produce the final gray level. Suppose that, inorder to maintain DC balance in this example, a net voltage impulse oftwo positive voltage frames need to be applied. The desired (target)gray level could be achieved by using three scan frames of impulse;however, in doing so, the system would be left DC imbalanced by oneframe. On the other hand, DC balance could be achieved by using twopositive voltage scan frames instead of three, but the final opticalstate will deviate significantly from the target.

One way to achieve DC balance is to use two positive voltage frames todrive the electro-optic medium to the vicinity of the desired graylevel, and also use a DC balanced FT sequence (an FT sequence that haszero net voltage impulse) to make the final adjustment sufficientlyclose to the target gray level, as illustrated symbolically in FIG. 28,in which the target gray level is achieved using two scan framesfollowed by an FT sequence of zero net voltage impulse chosen to givethe proper change in optical state.

Alternatively, one could use three positive voltage scan frames ofmonopolar drive to bring the reflectance to the target optical state,then use an FT sequence that has a net DC imbalance equivalent to onenegative voltage scan frame. If one chooses an FT sequence that resultsin a substantially unchanged optical state, then the final optical statewill remain correct and DC-balance will be restored. This example isshown in FIG. 29. It will be appreciated that typically use of FTsequences will involve some adjustment of optical state along with someeffect on DC balance, and that the above two examples illustrate extremecases.

The following Example is now given, though by way of illustration only,to show experimental uses of FT sequences in accordance with the presentinvention.

Example Use of FT Sequences in Cyclic RSGS Waveform

This Example illustrates the use of FT sequences in improving theoptical performance of a waveform designed at achieve 4 gray level(2-bit) addressing of a single pixel display. This display used anencapsulated electrophoretic medium and was constructed substantially asdescribed in Paragraphs [0069] to [0076] of the aforementioned2002/0180687. The single-pixel display was monitored by a photodiode.

Waveform voltages were applied to the pixel according to a transitionmatrix (look-up table), in order to achieve a sequence of gray levelswithin the 2-bit (4-state) grayscale. As already explained, a transitionmatrix or look-up table is simply a set of rules for applying voltagesto the pixel in order to make a transition from one gray level toanother within the gray scale.

The waveform was subject to voltage and timing constraints. Only threevoltage levels, −15V, 0V and +15V were applied across the pixel. Also,in order to simulate an active matrix drive with 50 Hz frame rate,voltages were applied in 20 ms increments. Tuning algorithms wereemployed iteratively in order to optimize the waveform, i.e. to achievea condition where the spread in the actual optical state for each of thefour gray levels across a test sequence was minimized.

In an initial experiment, a cyclic rail-stabilized grayscale (cRSGS)waveform was optimized using simple saturation and set pulses.Consideration of prior states was limited to the initial (R2) anddesired final (R1) gray levels in determining the transition matrix. Thewaveform was globally DC balanced. Because of the coarseness of theminimum impulse available for tuning (20 ms at 15 V), and the absence ofstates prior to R2 in the transition matrix, quite poor performance wasanticipated from this waveform.

The performance of the transition matrix was tested by switching thetest pixel through a “pentad-complete” gray level sequence, whichcontained all gray level pentad sequences in a random arrangement.(Pentad sequence elements are sequences of five gray levels, such as0-1-0-2-3 and 2-1-3-0-3, where 0, 1, 2 and 3 represent the four graylevels available.) For a perfect transition matrix, the reflectivity ofeach of the four gray levels is exactly the same for all occurrences ofthat gray level in the random sequence. The reflectivity of each of thegray levels will vary significantly for realistic transition matrices.The bar graph of FIG. 30 indeed shows the poor performance of thevoltage and timing limited transition matrix. The measured reflectivityof the various occurrences of each of the target gray levels is highlyvariable. The cRSGS waveform optimized without FT sequences developed inthis part of the experiment is hereinafter referred to as the basewaveform.

FT sequences were then incorporated into the cRSGS waveform; in thisexperiment, the FT sequences were limited to five scan frames, andincluded only DC balanced FT sequences. The FT sequences were placed atthe end of the base waveform for each transition, i.e., the waveform foreach transition had one of the following forms:

set-N

sat-set-N

sat-sat′-set-N.

Successful incorporation of FT sequence elements into the waveformrequired two steps; first, ascertaining the effect of various FTsequences on the optical state at each gray level and second selectingFT sequences to append to the various waveform elements.

To ascertain the effect of various FT sequences on the optical state ofeach gray level, an “FT efficacy” experiment was performed. First, aconsistent starting point was established by switching theelectrophoretic medium repeatedly between black and white optical rails.Then, the film was taken to one of the four gray levels (0, 1, 2, or 3),here referred to as the optical state R2. Then, the base waveformappropriate to make the transition from R2 to one of the other graylevels (here called R1) with an appended FT sequence was applied. Thisstep was repeated with all of the 51 DC balanced, 5-frame FT sequences.The final optical state was recorded for each of the FT sequences. TheFT sequences were then ordered according to their associated finalreflectivity. This process was repeated for all combinations of initial(R2) and final (R1) gray levels. The ordering of FT sequences for thefinal gray level 1 (R1=1) and the current gray level 0, 2 and 3 (R2=0,2, 3) are shown in Tables 7-9, respectively, where the columns labeled“Frame 1” to “Frame 5” show the potential in volts applied during thefive successive frames of the relevant FT sequence. The final opticalstates achieved for the waveform using the various FT sequences areplotted in FIG. 31. From this Figure, it will be seen that FT sequencescan be used to affect a large change in the final optical state, andthat the choices of five-scan-frame FT sequences afforded fine controlover the final optical state, all with no net voltage impulsedifference.

TABLE 7 Final optical states for gray level 0 to 1 for various FTsequences. Index Optical Number (L*) Frame 1 Frame 2 Frame 3 Frame 4Frame 5 1 35.13 0 15 15 −15 −15 2 35.20 15 0 15 −15 −15 3 35.22 15 15 0−15 −15 4 35.48 15 15 −15 −15 0 5 35.65 15 15 −15 0 −15 6 36.07 0 15 −1515 −15 7 36.10 15 −15 0 15 −15 8 36.23 15 0 −15 15 −15 9 36.26 15 −15 150 −15 10 36.32 15 −15 15 −15 0 11 36.34 −15 0 15 15 −15 12 36.36 −15 150 15 −15 13 36.37 0 0 15 0 −15 14 36.42 0 15 0 0 −15 15 36.47 0 0 0 15−15 16 36.51 −15 15 15 0 −15 17 36.51 0 15 0 −15 0 18 36.55 0 0 15 −15 019 36.59 −15 15 15 −15 0 20 36.59 0 15 −15 0 0 21 36.59 0 −15 15 15 −1522 36.68 15 0 0 0 −15 23 36.73 15 −15 −15 0 15 24 36.76 15 0 0 −15 0 2536.79 15 0 −15 0 0 26 36.86 0 15 −15 −15 15 27 36.87 15 −15 0 0 0 2837.00 15 0 −15 −15 15 29 37.03 −15 0 0 0 15 30 37.05 15 −15 −15 15 0 3137.11 −15 0 0 15 0 32 37.19 15 −15 0 −15 15 33 37.19 −15 15 −15 0 15 3437.22 0 −15 0 0 15 35 37.24 −15 0 15 0 0 36 37.26 −15 0 15 −15 15 3737.33 0 −15 0 15 0 38 37.43 0 0 −15 0 15 39 37.43 −15 15 −15 15 0 4037.49 −15 −15 15 0 15 41 37.50 −15 15 0 0 0 42 37.53 −15 15 0 −15 15 4337.55 0 −15 15 −15 15 44 37.58 0 −15 15 0 0 45 37.61 0 0 −15 15 0 4637.62 −15 −15 0 15 15 47 37.69 0 0 0 −15 15 48 37.72 0 0 0 0 0 49 37.85−15 −15 15 15 0 50 37.96 −15 0 −15 15 15 51 37.99 0 −15 −15 15 15

TABLE 8 Final optical states for gray level 2 to 1 for various FTsequences. Index Optical Number (L*) Frame 1 Frame 2 Frame 3 Frame 4Frame 5 1 34.85 0 15 15 −15 −15 2 34.91 15 0 15 −15 −15 3 35.07 15 15−15 −15 0 4 35.15 15 15 0 −15 −15 5 35.35 15 15 −15 0 −15 6 35.43 0 15−15 15 −15 7 35.46 15 −15 0 15 −15 8 35.51 0 0 15 −15 0 9 35.52 0 15 −150 0 10 35.52 0 0 0 15 −15 11 35.61 15 −15 15 −15 0 12 35.62 0 0 15 0 −1513 35.63 15 −15 0 0 0 14 35.65 −15 15 0 15 −15 15 35.67 0 15 0 −15 0 1635.70 −15 0 15 15 −15 17 35.75 15 −15 15 0 −15 18 35.76 0 15 0 0 −15 1935.77 15 0 −15 0 0 20 35.78 15 0 −15 15 −15 21 35.80 −15 15 15 −15 0 2235.97 −15 15 15 0 −15 23 35.98 15 0 0 −15 0 24 36.00 0 −15 15 15 −15 2536.06 0 0 0 0 0 26 36.09 −15 0 0 15 0 27 36.10 −15 0 0 0 15 28 36.10 150 0 0 −15 29 36.14 −15 0 15 0 0 30 36.28 −15 15 0 0 0 31 36.38 15 −15−15 0 15 32 36.40 0 15 −15 −15 15 33 36.41 0 −15 0 0 15 34 36.44 0 −15 015 0 35 36.45 15 −15 −15 15 0 36 36.49 −15 15 −15 0 15 37 36.49 0 −15 150 0 38 36.55 −15 0 15 −15 15 39 36.57 −15 15 −15 15 0 40 36.59 0 0 −15 015 41 36.63 0 0 −15 15 0 42 36.72 15 −15 0 −15 15 43 36.72 15 0 −15 −1515 44 36.77 0 0 0 −15 15 45 36.81 −15 15 0 −15 15 46 36.89 0 −15 15 −1515 47 36.98 −15 −15 15 0 15 48 37.16 −15 −15 15 15 0 49 37.19 −15 −15 015 15 50 37.42 −15 0 −15 15 15 51 37.51 0 −15 −15 15 15

TABLE 9 Final optical states for gray level 3 to 1 for various FTsequences. Index Optical Number (L*) Frame 1 Frame 2 Frame 3 Frame 4Frame 5 1 36.86 0 15 15 −15 −15 2 36.92 15 0 15 −15 −15 3 37.00 15 15−15 −15 0 4 37.13 15 15 0 −15 −15 5 37.39 15 15 −15 0 −15 6 37.47 0 15−15 15 −15 7 37.48 15 −15 0 15 −15 8 37.50 0 15 −15 0 0 9 37.52 0 0 15−15 0 10 37.53 0 0 0 15 −15 11 37.60 15 −15 15 −15 0 12 37.62 15 −15 0 00 13 37.63 0 0 15 0 −15 14 37.65 0 15 0 −15 0 15 37.67 −15 15 0 15 −1516 37.71 −15 0 15 15 −15 17 37.76 0 15 0 0 −15 18 37.77 15 −15 15 0 −1519 37.79 15 0 −15 15 −15 20 37.80 15 0 −15 0 0 21 37.82 −15 15 15 −15 022 37.96 15 0 0 −15 0 23 38.01 −15 15 15 0 −15 24 38.03 0 −15 15 15 −1525 38.04 0 0 0 0 0 26 38.09 −15 0 0 15 0 27 38.09 15 0 0 0 −15 28 38.15−15 0 0 0 15 29 38.16 −15 0 15 0 0 30 38.24 −15 15 0 0 0 31 38.40 15 −15−15 0 15 32 38.43 0 −15 0 0 15 33 38.44 0 −15 0 15 0 34 38.44 0 15 −15−15 15 35 38.46 15 −15 −15 15 0 36 38.51 −15 15 −15 0 15 37 38.52 0 −1515 0 0 38 38.59 −15 0 15 −15 15 39 38.61 −15 15 −15 15 0 40 38.65 0 0−15 0 15 41 38.66 0 0 −15 15 0 42 38.74 15 0 −15 −15 15 43 38.74 15 −150 −15 15 44 38.82 0 0 0 −15 15 45 38.89 −15 15 0 −15 15 46 38.95 0 −1515 −15 15 47 39.02 −15 −15 15 0 15 48 39.21 −15 −15 15 15 0 49 39.22 −15−15 0 15 15 50 39.44 −15 0 −15 15 15 51 39.53 0 −15 −15 15 15

Next, a cRSGS waveform was constructed using FT sequences chosen usingthe results represented in Tables 7 to 9 and FIG. 31 (specificallySequence 33 from Table 7, Sequence 49 from Table 8 and Sequence 4 fromTable 9), and their analogs for the other final gray levels. It is notedthat the region between 36.9 and 37.5 L* on the y-axis in FIG. 31 showsthe overlap between optical reflectance of the same final (R1) statewith different initial (R2) states made available by using DC balancedFT sequences. Therefore, a target gray level for R1=1 was chosen at 37.2L*, and the FT sequence for each R2 that gave the final optical stateclosest to this target was selected. This process was repeated for theother final optical states (R1=0, 2 and 3).

Finally, the resultant waveform was tested using the pseudo-randomsequence containing all five-deep state histories that was describedearlier. This sequence contains 324 transitions of interest. The cRSGSwaveform modified by the selected FT sequences was used to achieve allthe transitions in this sequence, and the reflectivity of each of theoptical states achieved was recorded. The optical states achieved areplotted in FIG. 32. It is apparent by comparing FIG. 32 with FIG. 30that the spread in reflectivity of each of the gray levels was greatlyreduced by incorporation of the FT sequences.

In summary, non-contiguous addressing provides FT sequences which either(i) allow changes in the optical state or (ii) allow a means ofachieving DC balance, or at least a change in the degree of DCimbalance, of a waveform. As already noted, it is possible to give arather mathematical definition of an FT sequence, for example, for theDC imbalanced variant of the method:

(a) Application of a DC imbalanced FT sequence that results in a changein optical state that is substantially different from the change inoptical state of its DC reference pulse. The “DC reference pulse” is apulse of voltage V0, where V0 is the voltage corresponding to themaximum voltage amplitude applied during the FT sequence but with thesame sign as the net impulse of the FT sequence. The net impulse of asequence is the area under the voltage versus time curve, and is denotedby the symbol G. The duration of the reference pulse is T=G/V0. This FTsequence is utilized to introduce a DC imbalance that differssignificantly from the net DC imbalance of its reference pulse.

(b) Application of a DC imbalanced FT sequence that results in a changein optical state that is much smaller in magnitude than the opticalchange one would achieve with its time reference pulse. The“time-reference pulse” is defined as a single-signed-voltage pulse ofthe same duration as the FT sequence, but where the sign of thereference pulse is chosen to give the largest change in optical state.That is, when the electro-optic medium is near its white state, anegative voltage pulse may drive the electro-optic medium only slightlymore white, whereas a positive voltage may drive the electro-opticmedium strongly toward black. The sign of the reference pulse in thiscase is positive. The goal of this type of FT pulse is to adjust the netvoltage impulse (for DC balancing, for example) while not stronglyaffecting the optical state.

Non-contiguous addressing also relates to the concept of using one ormore FT sequences between or inserted into pulse elements of atransition waveform, and to the concept of using FT sequences to balanceagainst the effect of prior gray levels and prior dwell times Onespecific example of the present invention is the use of zero voltageframes inserted in the middle of a pulse element of a waveform or inbetween pulse elements of a waveform to change the final optical state.

Non-contiguous addressing also allows fine tuning of waveforms toachieve desired gray levels with desired precision, and a means by whicha waveform can be brought closer to DC balanced (that is, zero netvoltage impulse for any cyclic excursion to various gray levels), usingsource drivers that do not permit fine tuning of the voltage, especiallysource drivers with only two or three voltage levels.

Part B: DC Balanced Addressing Method

The sawtooth (cRSGS) drive scheme described above with reference toFIGS. 11A and 11B is well adapted for use in DC balancing, in that thisdrive scheme ensures that only a limited number of transitions canelapse between successive passes of any given pixel though the blackstate, and indeed that on average a pixel will pass through the blackstate on one-half of its transitions.

However, DC balancing is not confined to balancing the aggregate of theimpulses applied to the electro-optic medium during a succession oftransitions, but also extends to making at least some of the transitionsundergone by the pixels of the display “internally” DC balanced, as willnow be described in detail.

DC balanced transitions are advantageous for driving encapsulatedelectrophoretic and other impulse-driven electro-optic media for displayapplications. Such transitions can be applied, for example, to anactive-matrix display that has source drivers that can output only twoor three voltages. Although other types of drivers can be used, most ofthe detailed description below will focus on examples using sourcedrivers with three voltage outputs (positive, negative, and zero).

In the following description of a DC balanced addressing method, as inthe preceding description of other aspects of the invention, the graylevels of an electro-optic medium will be denoted 1 to N, where 1denotes the darkest state and N the lightest state. The intermediatestates are numbered increasing from darker to lighter. A drive schemefor an impulse driven imaging medium makes use of a set of rules forachieving transitions from an initial gray level to a final gray level.The drive scheme can be expressed as a voltage as a function of time foreach transition, as shown in Table 10 for each of the 16 possibletransitions in a 2-bit (4 gray level) gray scale display.

TABLE 10 final gray level 1 2 3 4 initial gray 1 V₁₁(t) V₁₂(t) V₁₃(t)V₁₄(t) level 2 V₂₁(t) V₂₂(t) V₂₃(t) V₂₄(t) 3 V₃₁(t) V₃₂(t) V₃₃(t) V₃₄(t)4 V₄₁(t) V₄₂(t) V₄₃(t) V₄₄(t)

In Table 10, V_(ij)(t) denotes the waveform used to make the transitionfrom gray level i to gray level j. DC-balanced transitions are oneswhere the time integral of the waveform V_(ij)(t) is zero.

The term “optical rails” has already been defined above as meaning theextreme optical states of an electro-optic medium. The phrase “pushingthe medium towards or into an optical rail” will be employed below. By“towards”, is meant that a voltage is applied to move the optical stateof the medium toward one of the optical rails. By “pushing”, is meantthat the voltage pulse is of sufficient duration and amplitude that theoptical state of the electro-optic medium is brought substantially closeto one of the optical rails. It is important to note that “pushing intoan optical rail” does not mean that the optical rail state isnecessarily achieved at the end of the pulse, but that an optical statesubstantially close to the final optical state is achieved at the end ofthe pulse. For example, consider an electro-optic medium with opticalrails at 1% and 50% reflectivities. A 300 msec pulse was found to bringthe final optical state (from 1% reflectivity) to 50% reflectivity. Onemay speak of a 200 msec pulse as pushing the display into thehigh-reflectivity optical rail even though it achieves a finalreflectivity of only 45% reflectance. This 200 msec pulse is thought ofas pushing the medium into one of the optical rails because the 200 msecduration is long compared to the time required to traverse a largefraction of the optical range, such as the middle third of the opticalrange (in this case, 200 msec is long compared to the pulse required tobring the medium across the middle third of the reflectivity range, inthis case, from 17% to 34% reflectance).

Three different types of DC balanced transitions will now be described,together with a hybrid drive scheme using both DC balanced and DCimbalanced transitions. In the following description for conveniencepulses will a denoted by a number, the magnitude of the numberindicating the duration of the pulse. If the number is positive, thepulse is positive, and if the number is negative, the pulse is negative.Thus, for example, if the available voltages are +15V, 0V, and −15V, andthe pulse duration is measured in milliseconds (msec), then a pulsecharacterized by x=300 indicates a 300 msec, 15V pulse, and x=−60indicates a 60 msec, −15V pulse.

Type I:

In the first and simplest type of DC balanced transition, a voltagepulse (“x”) is preceded by a pulse (“−x”) of equal length but ofopposite sign, as illustrated in FIG. 33. (Note that the value of x canitself be negative, so the positive and negative pulses may appear inthe opposite order from that shown in FIG. 33.)

As mentioned above, it has been found that the effect of the waveformused to effect a transition is modified by the presence of a period ofzero voltage (in effect a time delay) during or before any of the pulsesin the waveform, in accordance with the non-contiguous addressing methodof the present invention. FIGS. 34 and 35 illustrate modifications ofthe waveform of FIG. 33. In FIG. 34, a time delay is inserted betweenthe two pulses of FIG. 33 while in FIG. 35 the time delay in insertedwithin the second pulse of FIG. 33, or, which amounts to the same thing,the second pulse of FIG. 33 is split into two separate pulses separatedby the time delay. As already described, time delays can be incorporatedinto a waveform to achieve optical states not achievable without suchdelays. Time delays can also be used to fine-tune the final opticalstate. This fine-tuning ability is important, because in an activematrix drive, the time resolution of each pulse is defined by the scanrate of the display. The time resolution offered by the scan rate can becoarse enough that precise final optical states cannot be achievedwithout some additional means of fine tuning. While time delays offer asmall degree of fine tuning of the final optical state, additionalfeatures such as those described below offer additional means of coarseand fine tuning of the final optical state.

Type II

A Type II waveform consists of a Type I waveform as described above withthe insertion of a positive and negative pulse pair (denoted “+y” and“−y” pulses) at some point into the Type I waveform, as indicatedsymbolically in FIG. 36. The +y and −y pulses do not have to beconsecutive, but can be present at different places into the originalwaveform. There are two especially advantageous forms of the Type IIwaveform.

Type II: Special Case A:

In this special form, the “−y,+y” pulse pair is placed before the“−x,+x” pulse pair. It has been found that, when y and x are of oppositesign, as illustrated in FIG. 37, the final optical state can be finelytuned by even moderately coarse adjustment of the duration y. Thus, thevalue of x can be adjusted for coarse control and the value of y forfinal control of the final optical state of the electro-optic medium.This is believed to happen because the y pulse augments the −x pulse,thus changing the degree to which the electro-optic medium is pushedinto one of its optical rails. The degree of pushing into one of theoptical rails is known to give fine adjustment of the final opticalstate after a pulse away from that optical rail (in this case, providedby the x pulse).

Type II: Special Case B:

For reasons indicated above, it has been found advantageous to usewaveforms with at least one pulse element long enough to drive theelectro-optic medium substantially into one optical rail. Also, for amore visually pleasing transition, it is desirable to arrive to thefinal optical state from the nearer optical rail, since achieving graylevels near an optical rail requires only a short final pulse. Waveformsof this type require at least one long pulse for driving into an opticalrail and a short pulse to achieve the final optical state near thatoptical rail, and hence cannot have the Type I structure describedabove. However, special cases of the Type II waveform can achieve thistype of waveform. FIG. 38 shows one example of such a waveform, wherethe +y pulse is placed after the −x,+x pulse pair and the −y pulse isplaced before the −x,+x pulse pair. In this type of waveform, the final+y pulse provides coarse tuning because the final optical state is verysensitive to the magnitude of y. The +x pulse provides a finer tuning,since the final optical state typically does not depend as strongly onthe magnitude of the drive into the optical rail.

Type III:

A third type (Type III) of DC balanced transition introduces yet anotherDC-balanced pulse pair (denoted “−z”, “+z”) into the waveform, as shownschematically in FIG. 39. A preferred example of such a Type IIIwaveform is shown in FIG. 40; this type of waveform is useful for finetuning of the final optical state, for the following reasons. Considerthe situation without the +z and −z pulses (i.e. the Type II waveformdiscussed above). The x pulse element is used for fine tuning, and thefinal optical state can be decreased by increasing x and increased bydecreasing x. However, it is undesirable to decrease x beyond a certainpoint because then the electro-optic medium is not brought sufficientlyclose to an optical rail, as required for stability of the waveform. Toavoid this problem, instead of decreasing x, one can (in effect)increase the −x pulse without changing the x pulse by adding the −z,+zpulse pair as shown in FIG. 40, with z having the opposite sign from x.The z pulse augments the −x pulse, while the −z pulse maintains thetransition at zero net impulse, i.e., maintains a DC-balancedtransition.

The Type I, II and III waveforms discussed above can of course bemodified in various ways. Additional pairs of pulses can be added to thewaveform to achieve more general structures. The advantage of suchadditional pairs diminishes with increasing number of pulse elements,but such waveforms are a natural extension of the Type I, II and IIIwaveforms. Also, as already discussed, one or more time delays can beinserted in various places in any of the waveforms, in the same manneras illustrated in FIGS. 34 and 35. As mentioned earlier, time delays inpulses affect the final optical state achieved, and are thus useful forfine tuning. Also, the placement of time delays can change the visualappearance of transitions by changing the position of transitionelements relative to other elements in the same transition as well asrelative to transition elements of other transitions. Time delays canalso be used to align certain waveform transition elements, and this maybe advantageous for some display modules with certain controllercapabilities. Also, in recognition of the fact that small changes in theordering of the applied pulses may substantially change the opticalstate following the pulses, the output signal may also be formed bytransposing all or part of one of the above-described pulse sequences,or by repeated transpositions of all or part of one of the abovedescribed sequences, or by the insertion of one or more 0 V periods atany location within one of the above-described sequences. In addition,these transposition and insertion operators can be combined in any order(e.g., insert 0 V section, then transpose, then insert 0 V section). Itis important to note that all such pulse sequences formed by thesetransformations retain the essential character of having zero netimpulse.

Finally, DC balanced transitions can be combined with DC imbalancedtransitions to form a complete drive scheme. For example, the −x/)IP/xwaveform described above and illustrated in FIG. 12, while satisfactoryfor transitions between differing optical states, is less satisfactoryfor zero transitions in which the initial and final optical states arethe same. For these zero transitions there is used, in this example, aType II waveform such as the ones shown in FIGS. 37 and 38. Thiscomplete waveform is shown symbolically in Table 11, from which it willbe seen that the −x/ΔIP/x waveform is used for non-zero transitions andthe Type II waveform for zero transitions.

TABLE 11 final gray level 1 2 3 4 initial gray 1 Type II −x/ΔIP/x−x/ΔIP/x −x/ΔIP/x level 2 −x/ΔIP/x Type II −x/ΔIP/x −x/ΔIP/x 3 −x/ΔIP/x−x/ΔIP/x Type II −x/ΔIP/x 4 −x/ΔIP/x −x/ΔIP/x −x/ΔIP/x Type II

The use of DC balanced transitions is not of course confined totransition matrices of this type, in which DC balanced transitions areconfined to the “leading diagonal” transitions, in which the initial andfinal gray levels are the same; to produce the maximum improvement incontrol of gray levels, it is generally desirable to maximize the numberof transitions which are DC balanced. However, depending upon thespecific electro-optic medium being used, it may be difficult to DCbalance transitions involving transitions to or from extreme graylevels, for example to or from black and white, gray levels 1 and 4respectively. Furthermore, in choosing which transitions are to be DCbalanced, it is important not to imbalance the overall transitionmatrix, i.e., to produce a transition matrix in which a closed loopbeginning and ending at the same gray level is DC imbalanced. Forexample, a rule that transitions involving only a change of 0 or 1 unitin gray level are DC balanced but other transitions are DC imbalanced isnot desirable, since this would imbalance the entire transition matrix,as shown by the following example; a pixel undergoing the sequence ofgray levels 2-4-3-2 would experience transitions 2-4 (DC imbalanced),4-3 (balanced) and 3-2 (balanced), so that the entire loop would beimbalanced. A practical compromise between these two conflicting desiresmay be to use DC balanced transitions in cases where only mid graylevels (levels 2 and 3) are involved and DC imbalanced transitions wherethe transition begins or ends at an extreme gray level (level 1 or 4).Obviously, the mid gray levels chosen for such a rule may vary with thespecific electro-optic medium and controller used; for example, inthree-bit (8 gray level) display it might be possible to use DC balancedtransitions in all transitions beginning or ending at gray levels 2-7(or perhaps 3-6) and DC imbalanced transitions in all transitionsbeginning or ending at gray levels 1 and 8 (or 1, 2, 7 and 8).

From the foregoing, it will be seen that the use of DC balancedtransitions allows fine tuning of waveforms to achieve desired graylevels with high precision, and provides a means by which a waveformtransition can have zero net voltage, using source drivers that do notpermit fine tuning of the voltage, especially source drivers with onlytwo or three voltage levels. It is believed that DC balanced waveformtransitions offer better performance than DC imbalanced waveforms. Thisinvention applies to displays in general, and especially, although notexclusively, to active-matrix display modules with source drivers thatoffer only two or three voltages. This invention also applies toactive-matrix display modules with source drivers that offer morevoltage levels.

The use of DC balanced transitions can provide certain additionaladvantages. As noted above, in some driving methods of the invention,the transition matrix is a function of variables other than prioroptical state, for example the length of time since the last update, orthe temperature of the display medium. It is quite difficult to maintainDC balance in these cases with non-balanced transitions. For example,consider a display that repeatedly transitions from white to black at25° C. and then from black to white at 0° C. The slower response at lowtemperature will typically dictate using a longer pulse length. As aresult, the display will experience a net DC imbalance towards white. Onthe other hand, if all transitions are internally balanced, thendifferent transition matrices can be freely mixed without introducing DCimbalance.

Part C: Defined Region Method

The objectionable effects of reset steps, as described above, may befurther reduced by using local rather than global updating, i.e., byrewriting only those portions of the display which change betweensuccessive images, the portions to be rewritten being chosen on either a“local area” or a pixel-by-pixel basis. For example, it is not uncommonto find a series of images in which relatively small objects move acrossa larger static background, as for example in diagrams illustratingparts in mechanical devices or diagrams used in accident reconstruction.To use local updating, the display controller needs to compare the finalimage with the initial image and determine which area(s) differ betweenthe two images and thus need to be rewritten. The controller mayidentify one or more local areas, typically rectangular areas havingaxes aligned with the pixel grid, which contain pixels which need to beupdated, or may simply identify individual pixels which need to beupdated. Any of the drive schemes already described may then be appliedto update only the local areas or individual pixels thus identified asneeding rewriting. Such a local updating scheme can substantially reducethe energy consumption of a display.

Use of a “defined region” updating method of this type permits updatingof a bistable electro-optic display using different updating methods indifferent regions of the display.

Electro-optic displays are known in which the entire display can bedriven in a one-bit or in a grayscale mode. When the display is inone-bit mode, updates are effected using a one-bit general image flow(GIF) waveform, whereas when the display is in grayscale mode, updatesare effected using a multi-prepulse slide show waveform, or some otherslow waveform, even if, in a specific area of the display, only one-bitinformation is being updated.

Such an electro-optic display may be modified to carry out a definedregion updating method by defining two additional commands in thecontroller, namely a “DEFINE REGION” command and a “CLEAR ALL REGIONS”command. The DEFINE REGION command typically takes as argumentslocations sufficient to define completely a rectangular area of thedisplay, for example the locations of the upper right and lower leftcorners of the defined region; this command may also have an additionalargument specifying the bit depth to which the defined region is set,although this last argument is not necessary in simple forms of thedefined region method in which the defined region is always monochrome.The bit depth set by the last argument of course overrides any bit depthpreviously set for the defined region. Alternatively, the DEFINE REGIONcommand could specify a series of points defining the vertices of apolygon. The CLEAR ALL REGIONS command may take no arguments and simplyreset the entire display to a single predefined bit depth, or might takea single argument specifying which of various possible bit depths is tobe adopted by the entire display after the clearing operation.

It will be appreciated that a defined region method is not restricted tothe use of only two regions and more regions could be provided ifdesired. For example, in an image editing program it might be helpful tohave a main region showing the image being edited at full bit depth, andboth an information display region (for example, a box showing presentcursor position) and a dialog box region (providing a dialog box forentry of text by the user) running in one-bit mode. The defined regionmethod will primarily be described below in a two-region version, sincethe necessary modifications to enable use of more than two regions willreadily be apparent to those skilled in the construction of displaycontrollers.

In order to keep track of the depths of the different regions, thecontroller may keep an array of storage elements, one element beingassociated with each pixel in the display, and each element storing avalue representing the current bit depth for the associated pixel. Forexample, an SVGA (800×600) display capable of operating in either 1-bitor 2-bit mode could use an 800×600 array of 1-bit elements (eachcontaining 0 for 1-bit mode, 1 for 2-bit mode). In such a controller,the DEFINE REGION command would set the elements within the definedregion of the display to the requested bit depth, while the CLEAR ALLREGIONS command would reset all elements of the array to the same value(either a predetermined value or one defined by the argument of thecommand).

Optionally, when a region is defined or cleared, the controller couldexecute an update sequence on the pixels within that region to transferthe display from one mode to the other, in order to ensure DC balancingor to adjust the optical states of the relevant pixels, for example byusing an FT sequence as described above.

When a display is operating in defined region mode, a new image is sentto the controller, and the display must be redrawn, there are threepossible cases:

-   -   1. Only pixels within the defined (say) one-bit region have        changed. In this case, a one-bit (fast) waveform can be used to        update the display;    -   2. Only pixels within the non-defined (grayscale) regions have        changed. In this case, a grayscale (slow) waveform must be used        to update the display (note that since by definition not pixels        are changed within the defined region, the legibility of the        defined region, for example a dialog box, during the redrawing        is not a problem); and    -   3. Pixels in both the defined and non-defined regions have        changed. In this case, the grayscale pixels are updated using        the grayscale waveform, and the one-bit pixels are updated using        the one-bit waveform (the shorter one-bit waveforms must be        zero-padded appropriately to match the length of the grayscale        update).

The controller may determine, before scanning the display, which ofthese cases exists by performing the following logical tests (assuming aone-bit value associated with each pixel and storing the pixel mode, asdefined above):

-   -   (Old_image XOR new_image)>0: pixels are changed in the display    -   (Old_image XOR new_image) AND mode_array>0: grayscale pixels are        changed    -   (Old_image XOR new_image) AND (NOT mode_array)>0: monochrome        pixels are changed

As the controller scans the display, for case 1 or case 2 it can use onewaveform look-up table for all pixels, since the unchanged pixels willreceive 0 V, assuming that a null transition in one-bit mode is the sameas in grayscale mode (in other words, that both waveforms arelocal-update). If instead the grayscale waveform is global-update (allpixels are updated whenever the display is updated), then the controllerwill need to test to see if a pixel is within the appropriate region todetermine whether to apply the global-update waveform or not. For Case3, the controller must check the value of the mode bit array for eachpixel as it scans to determine which waveform to use.

Optionally, if the lightness values of the black and white statesachieved in one-bit mode are identical to those achieved in grayscalemode, in Case 3 above the grayscale waveform can be used for all pixelsin the display, thus eliminating the need for transfer functions betweenthe one-bit and grayscale waveforms.

The defined region method may make use of any of the optional featuresof the basic look-up table method, as described above.

The primary advantage of the defined region method is that it enablesthe use of a fast one-bit waveform on a display that is displaying apreviously written grayscale image. Prior art display controllerstypically only allow the display to be in either grayscale or one-bitmode at any one time. While it is possible to write one-bit images ingrayscale mode, the relevant waveforms are quite slow. In addition, thedefined region method is essentially transparent to the host system (thesystem, typically a computer) which supplies images to the controller,since the host system does not have to advise the controller whichwaveform to use. Finally, the defined region method allows both one-bitand grayscale waveforms to be used on the display at the same time,whereas other solutions require two separate update events if both kindsof waveforms are being used.

The aforementioned drive schemes may be varied in numerous waysdepending upon the characteristics of the specific electro-optic displayused. For example, in some cases it may be possible to eliminate many ofthe reset steps in the drives schemes described above. For example, ifthe electro-optic medium used is bistable for long periods (i.e., thegray levels of written pixels change only very slowly with time) and theimpulse needed for a specific transition does not vary greatly with theperiod for which the pixel has been in its initial gray state, a look-uptable may be arranged to effect gray state to gray state transitionsdirectly without any intervening return to a black or white state,resetting of the display being effected only when, after a substantialperiod has elapsed, the gradual “drift” of pixels from their nominalgray levels has caused noticeable errors in the image presented. Thus,for example, if a user was using a display of the present invention asan electronic book reader, it might be possible to display numerousscreens of information before resetting of the display were necessary;empirically, it has been found that with appropriate waveforms anddrivers, as many as 1000 screens of information can be displayed beforeresetting is necessary, so that in practice resetting would not benecessary during a typical reading session of an electronic book reader.

It will readily be apparent to those skilled in display technology thata single apparatus of the present invention could usefully be providedwith a plurality of different drive schemes for use under differingconditions. For example, since in the drive schemes shown in FIGS. 9 and10, the reset pulses consume a substantial fraction of the total energyconsumption of the display, a controller might be provided with a firstdrive scheme which resets the display at frequent intervals, thusminimizing gray scale errors, and a second scheme which resets thedisplay only at longer intervals, thus tolerating greater gray scaleerrors but reduce energy consumption. Switching between the two schemescan be effected either manually or dependent upon external parameters;for example, if the display were being used in a laptop computer, thefirst drive scheme could be used when the computer is running on mainselectricity, while the second could be used while the computer wasrunning on internal battery power.

Part D: Compensation Voltage Method

The methods of the present invention can be used in combination with a“compensation voltage” method and apparatus, which will now be describedin detail.

The compensation voltage method and apparatus seek to achieve resultssimilar to the basic look-up table methods described above without theneed to store very large look-up tables. The size of a look-up tablegrows rapidly with the number of prior states with regard to which thelook-up table is indexed. For this reason, as already discussed, thereis a practical limitation and cost consideration to increasing thenumber of prior states used in choosing an impulse for achieving adesired transition in a bistable electro-optic display.

In the compensation voltage method and apparatus, the size of thelook-up table needed is reduced, and compensation voltage data is storedfor each pixel of the display, this compensation voltage data beingcalculated dependent upon at least one impulse previously applied to therelevant pixel. The voltage finally applied to the pixel is the sum of adrive voltage, chosen in the usual way from the look-up table, and acompensation voltage determined from the compensation voltage data forthe relevant pixel. In effect, the compensation voltage data applies tothe pixel a “correction” such as would otherwise be applied by indexingthe look-up table for one or more additional prior states.

The look-up table used in the compensation voltage method may be of anyof the types described above. Thus, the look-up table may be a simpletwo-dimensional table which allows only for the initial and final statesof the pixel during the relevant transition. Alternatively, the look-uptable may take account of one or more temporal and/or gray level priorstates. The compensation voltage may also take into account only thecompensation voltage data stored for the relevant pixel but mayoptionally also take into account of one or more temporal and/or graylevel prior states. The compensation voltage may be applied to therelevant pixel not only during the period for which the drive voltage isapplied to the pixel but also during so-called “hold” states when nodrive voltage is being applied to the pixel.

The exact manner in which the compensation voltage data is determinedmay vary widely with the characteristics of the bistable electro-opticmedium used. Typically, the compensation voltage data will periodicallybe modified in a manner which is determined by the drive voltage appliedto the pixel during the present and/or one or more scan frames. Inpreferred forms of the invention, the compensation voltage data consistsof a single numerical (register) value associated with each pixel of thedisplay.

In a preferred embodiment, scan frames are grouped into superframes inthe manner already described so that a display update can be initiatedonly at the beginning of a superframe. A superframe may, for example,consist of ten display scan frames, so that for a display with a 50 Hzscan rate, a display scan is 20 ms long and a superframe 200 ms long.During each superframe while the display is being rewritten, thecompensation voltage data associated with each pixel is updated. Theupdating consists of two parts in the following order:

-   -   (1) Modifying the previous value using a fixed algorithm        independent of the pulse applied during the relevant superframe;        and    -   (2) Increasing the value from step (1) by an amount determined        by the impulse applied during the relevant superframe.

In a particularly preferred embodiment, steps (1) and (2) are carriedout as follows:

-   -   (1) Dividing the previous value by a fixed constant, which is        conveniently two; and    -   (2) Increasing the value from step (1) by an amount proportional        to the total area under the voltage/time curve applied to the        electro-optic medium during the relevant superframe.

In step (2), the increase may be exactly or only approximatelyproportional to the area under the voltage/time curve during therelevant superframe. For example, as described in detail below withreference to FIG. 41, the increase may be “quantized” to a finite set ofclasses for all possible applied waveforms, each class including allwaveforms with a total area between two bounds, and the increase in step(2) determined by the class to which the applied waveform belongs.

The following example is now given. The display used was a two-bit grayscale encapsulated electrophoretic display, and the drive methodemployed used a two-dimensional look-up table as shown in Table 12below, which takes account only of the initial and final states of thedesired transition; in this Table, the column headings represent thedesired final state of the display and the row headings represent theinitial state, while the numbers in individual cells represent thevoltage in volts to be applied to the pixel for a predetermined period.

TABLE 12 to: to: to: to: 0 1 2 3 from: 0 0 +6 +9 +15 from: 1 −6 0 +6 +9from: 2 −9 −6 0 +6 from: 3 −15 −9 −6 0

To allow for practice of the compensation voltage method, a singlenumerical register was associated with each pixel of the display. Thevarious impulses shown in Table 12 were classified and a pulse class wasassociated with each impulse, as shown in Table 13 below.

TABLE 13 pulse voltage (V) −15 −9 −6 0 +6 +9 +15 pulse class −30 −18 −120 12 18 30

During each superframe, the numerical register associated with eachpixel was divided by 2, and then increased by the numerical value shownin Table 13 for the pulse being applied to the relevant pixel during thesame superframe. The voltage applied to each pixel during the superframewas the sum of the drive voltage, as shown in Table 12 and acompensation voltage, V_(comp), given by the formula:V _(comp) =A*(pixel register)where the pixel register value is read from the register associated withthe relevant pixel and “A” is a pre-defined constant.

In a laboratory demonstration of this preferred compensation voltagemethod, single pixel displays using an encapsulated electrophoreticmedium sandwiched between parallel electrodes, the front one of whichwas formed of ITO and light-transmissive, were driven by 300millisecond±15V square wave pulses between their black and white states.The display started in its white state, was driven black, then back towhite after a dwell time. It was found that the lightness of the finalwhite state was a function of dwell time, as shown in FIG. 41 of theaccompanying drawings. Thus, this encapsulated electrophoretic mediumwas sensitive to dwell time, with the L* of the white state varying byabout 3 units depending upon dwell time.

To show the effect of the compensation voltage method, the experimentwas repeated, except that a compensation voltage, consisting of anexponentially decaying voltage starting at the end of each drive pulse,was appended to each pulse. The applied voltage was the sum of the drivevoltage and the compensation voltage. As shown in FIG. 41, the whitestate for various dwell times in the case with the compensation voltagewas much more uniform than for the uncompensated pulses. Thus, thisexperiment demonstrated that use of such compensation pulses inaccordance with the present invention can greatly reduce the dwell timesensitivity of an encapsulated electrophoretic medium.

The compensation voltage method of the present invention may make use ofany of the optional features of the basic look-up table method describedabove.

From the foregoing description, it will be seen that the presentinvention provides methods for controlling the operation ofelectro-optic displays which allow accurate control of gray scalewithout requiring inconvenient flashing of the whole display to one ofits extreme states at frequent intervals. The present invention alsoallows for accurate control of the display despite changes in thetemperature and operating time thereof, while lowering the powerconsumption of the display. These advantages can be achievedinexpensively, since the necessary controllers can be constructed fromcommercially available components.

Part E: DTD Integral Reduction Method

As mentioned above, it has been found that, at least in some cases, theimpulse necessary for a given transition in a bistable electro-opticdisplay varies with the residence time of a pixel in its optical state,this phenomenon, which does not appear to have previously been discussedin the literature, hereinafter being referred to as “dwell timedependence” or “DTD”. Thus, it may be desirable or even in some cases inpractice necessary to vary the impulse applied for a given transition asa function of the residence time of the pixel in its initial opticalstate.

The phenomenon of dwell time dependence will now be explained in moredetail with reference to FIG. 42 of the accompanying drawings, whichshows the reflectance of a pixel a function of time for a sequence oftransitions denoted R3→R2→R1, where each of the Rk terms indicates agray level in a sequence of gray levels, with R's with larger indicesoccurring before R's with smaller indices. The transitions between R3and R2 and between R2 and R1 are also indicated. DTD is the variation ofthe final optical state R1 caused by variation in the time spent in theoptical state R2, referred to as the dwell time. The DTD integralreduction method provides a method for reducing dwell time dependencewhen driving bistable electro-optic displays.

Although the invention is in no way limited by any theory as to itsorigin, DTD appears to be, in large part, caused by remnant electricfields experienced by the electro-optic medium. These remnant electricfields are residues of drive pulses applied to the medium. It is commonpractice to speak of remnant voltages resulting from applied pulses, andthe remnant voltage is simply the scalar potential corresponding toremnant electric fields in the usual manner appropriate to electrostatictheory. These remnant voltages can cause the optical state of a displayfilm to drift with time. They also can change the efficacy of asubsequent drive voltage, thus changing the final optical state achievedafter that subsequent pulse. In this manner, the remnant voltage fromone transition waveform can cause the final state after a subsequentwaveform to be different from what it would be if the two transitionswere very separate from each other. By “very separate” is meantsufficiently separated in time so that the remnant voltage from thefirst transition waveform has substantially decayed before the secondtransition waveform is applied.

Measurements of remnant voltages resulting from transition waveforms andother simple pulses applied to an electro-optic medium indicate that theremnant voltage decays with time. The decay appears monotonic, but notsimply exponential. However, as a first approximation, the decay can beapproximated as exponential, with a decay time constant, in the case ofmost encapsulated electrophoretic media tested, of the order of onesecond, and other bistable electro-optic media are expected to displaysimilar decay times.

Accordingly, the DTD integral reduction method provides a method ofdriving a bistable electro-optic display having at least one pixel whichcomprises applying to the pixel a waveform V(t) such that:

$\begin{matrix}{J = {\int_{0}^{T}{{V(t)}{M\left( {T - t} \right)}\ {\mathbb{d}t}}}} & (1)\end{matrix}$(where T is the length of the waveform, the integral is over theduration of the waveform, V(t) is the waveform voltage as a function oftime t, and M(t) is a memory function that characterizes the reductionin efficacy of the remnant voltage to induce dwell-time-dependencearising from a short pulse at time zero) is less than about 1 volt sec.Desirably J is less than about 0.5 volt sec., and most desirably lessthan about 0.1 volt sec. In fact J should be arranged to be as small aspossible, ideally zero.

Waveforms can be designed that give very low values of J and hence verysmall DTD, by generating compound pulses. For example, a long negativevoltage pulse preceding a shorter positive voltage pulse (with a voltageamplitude of the same magnitude but of opposite sign) can result in amuch-reduced DTD. It is believed that the two pulses provide remnantvoltages with opposite signs. When the ratio of the lengths of the twopulses are correctly set, the remnant voltages from the two pulses canbe caused to largely cancel each other. The proper ratio of the lengthof the two pulses can be determined by the memory function for theremnant voltage.

In a presently preferred embodiment, J is calculated by:

$\begin{matrix}{J = {\int_{0}^{T}{{V(t)}{\exp\left( {- \frac{T - t}{\tau}} \right)}\ {\mathbb{d}t}}}} & (2)\end{matrix}$where τ is a decay (relaxation) time best determined empirically.

For some encapsulated electrophoretic media, it has been foundexperimentally that waveforms that give rise to small J values also giverise to particularly low DTD, while waveforms with particularly large Jvalues give rise to large DTD. In fact, good correlation has been foundbetween J values calculated by Equation (2) above with τ set to onesecond, roughly equal to the measured decay time of the remnant voltageafter an applied voltage pulse.

Thus, it is advantageous to use waveforms where each transition (or atleast most of the transitions in the look-up table) from one gray levelto another is achieved with a waveform that gives a small value of J.This J value is preferably zero, but empirically it has been found that,at least for the encapsulated electrophoretic media described in theaforementioned patents and application, as long as J had a magnitudeless than about 1 volt sec. at ambient temperature, the resulting dwelltime dependence is quite small.

Thus, one can provide a waveform for achieving transitions between a setof optical states, where, for every transition, a calculated value for Jhas a small magnitude. The J is calculated by a memory function that ispresumably monotonically decreasing. This memory function is notarbitrary but can be estimated by observing the dwell time dependence ofthe display film to simple voltage pulse or compound voltage pulses. Asan example, one can apply a voltage pulse to the display film to achievea transition from a first to a second optical state, wait a dwell time,then apply a second voltage pulse to achieve a transition from thesecond to a third voltage pulse. By monitoring the shift in the thirdoptical state as a function of the dwell time, one can determine anapproximate shape of the memory function. The memory function has ashape approximately similar to the difference in the third optical statefrom its value for long dwell times, as a function of the dwell time.The memory function would then be given this shape, and would haveamplitude of unity when its argument is zero. This method yields only anapproximation of the memory function, and for various final opticalstates, the measured shape of the memory function is expected to changesomewhat. However, the gross features, such as the characteristic timeof decay of the memory function, should be similar for various opticalstates. However, if there are significant differences in shape withfinal optical state, then the best memory function shape to adopt is onegained when the third optical state is in the middle third of theoptical range of the display medium. The gross features of the memoryfunction should also be estimable by measuring the decay of the remnantvoltage after an applied voltage pulse.

Although, the methods discussed here for estimating the memory functionare not exact, it has been found that J values calculated from even anapproximate memory are a good guide to waveforms having low DTD. Auseful memory function expresses the gross features of the timedependence of the DTD as described above. For example, a memory functionthat is exponential with a decay time of one second has been found towork well in predicting waveforms that gave low DTD. Changing the decaytime to 0.7 or 1.3 second does not destroy the effectiveness of theresulting J values as predictors of low DTD waveforms. However, a memoryfunction that does not decay, but remains at unity indefinitely, isnoticeably less useful as a predictor, and a memory function with a veryshort decay time, such as 0.05 second, was not a good predictor of lowDTD waveforms.

An example of a waveform that gives a small J value is the waveformshown in FIGS. 39 and 40 described above, where the x, y, and z pulsesare all of durations much smaller than the characteristic decay time ofthe memory function. This waveform functions well when this condition ismet because this waveform is composed of sequential opposing pulseelements whose remnant voltages tend to approximately cancel. For x andy values that are not much smaller than the characteristic decay time ofthe memory function but not larger than this decay time, it is foundthat that waveforms where x and y are of opposite sign tend to givelower J values, and x and y pulse durations can be found that actuallypermit very small J values because the various pulse elements giveremnant voltages that cancel each other out after the waveform isapplied, or at least largely cancel each other out.

It will be appreciated that the J value of a given waveform can bemanipulated by inserting periods of zero voltage into the waveform, oradjusting the lengths of any periods of zero voltage already present inthe waveform. Thus a wide variety of waveforms can be used while stillmaintaining a J value close to zero.

The DTD integral reduction method has general applicability. A waveformstructure can be devised described by parameters, its J valuescalculated for various values of these parameters, and appropriateparameter values chosen to minimize the J value, thus reducing the DTDof the waveform.

Part F: Remnant Voltage Method

It has been found that the extent of DC imbalance in an electrophoreticmedium used in a display can be ascertained by measuring theopen-circuit electrochemical potential (hereinafter for conveniencecalled the “remnant voltage” of the medium. When the remnant voltage ofa pixel is zero, it has been perfectly DC balanced. If its remnantvoltage is positive, it has been DC unbalanced in the positivedirection. If its remnant voltage is negative, it has been DC unbalancedin the negative direction. Remnant voltage data may be used to maintainlong-term DC balancing of the display.

In such a remnant voltage method, measurement of a remnant voltage isdesirably effected by a high impedance voltage measurement device, forexample a metal oxide semiconductor (MOS) comparator. When the displayis one having small pixels, for example a 100 dots per inch (DPI) matrixdisplay, in which each pixel has an area of 10⁻⁴ square inch or about6×10⁻² mm², the comparator needs to have an ultra-low input current, asthe resistance of such a single pixel is of the order of 10¹² ohm.However, suitable comparators are readily available commercially; forexample, the Texas Instruments INA 11 chip is suitable, as it has aninput current on only about 20 pA. (Technically, this integrated circuitis an instrumentation amplifier, but if its output is routed into aSchmitt trigger, it acts as a comparator.) For displays having largesingle pixels, such as large direct-drive displays (defined below) usedin signage, where the individual pixels may have areas of several squarecentimeters, the requirements for the comparator are much lessstringent, and almost any commercial FET input comparator may be used,for example the LF311 comparator from National SemiconductorCorporation.

It will readily be apparent to those skilled in the art of electronicdisplays that, for cost and other reasons, mass-produced electronicdisplays will normally have drivers in the form of application specificintegrated circuits (ASIC's), and in this type of display the comparatorwould typically be provided as part of the ASIC. Although this approachwould require provision of feedback circuitry within the ASIC, it wouldhave the advantage of making the power supply and oscillator sections ofthe ASIC simpler and smaller in area. If tri-level general image flowdrive is required, this approach would also make the driver section ofthe ASIC simpler and smaller in area. Thus, this approach wouldtypically reduce the cost of the ASIC.

Conveniently, a driver which can apply a driving voltage, electronicallyshort or float the pixel, is used to apply the driving pulses. Whenusing such a driver, on each addressing cycle where DC balancecorrection is to be effected, the pixel is addressed, electronicallyshorted, then floated. (The term “addressing cycle” is used herein inits conventional meaning in the art of electro-optic displays to referto the total cycle needed to change from a first to a second image onthe display. As noted above, because of the relatively low switchingspeeds of electrophoretic displays, which are typically of the order oftens to hundreds of milliseconds, a single addressing cycle may comprisea plurality of scans of the entire display.) After a short delay time,the comparator is used to measure the remnant voltage across the pixel,and to determine whether it is positive or negative in sign. If theremnant voltage is positive, the controller may slightly extend theduration of (or slightly increase the voltage of) negative-goingaddressing pulses on the next addressing cycle. If, however, the remnantvoltage is negative, the controller may slightly extend the duration of(or slightly increase the voltage of) positive-going voltage pulses onthe next addressing cycle.

Thus, the remnant voltage method places the electro-optic medium into abang-bang feedback loop, adjusting the length of the addressing pulsesto drive the remnant voltage toward zero. When the remnant voltage isnear zero, the medium exhibits ideal performance and improved lifetime.In particular, use of the present invention may allow improved controlof gray scale. As noted earlier, it has been observed that the grayscale level obtained in electro-optic displays is a function not only ofthe starting gray scale level and the impulse applied, but also of theprevious states of the display. It is believed that one of the reasonsfor this “history” effect on gray scale level is that the remnantvoltage affects the electric field experienced by the electro-opticmedium; the actual electric field influencing the behavior of the mediumis the sum of the voltage actually applied via the electrodes and theremnant voltage. Thus, controlling the remnant voltage ensures that theelectric field experienced by the electro-optic medium accuratelycorresponds to that applied via the electrodes, thus permitting improvedcontrol of gray scale.

The remnant voltage method is especially useful in displays of theso-called “direct drive” type, which are divided into a series of pixelseach of which is provided with a separate electrode, the display furthercomprising switching means arranged to control independently the voltageapplied to each separate electrode. Such direct drive displays areuseful for the display of text or other limited character sets, forexample numerical digits, and are described in, inter alia, theaforementioned International Application Publication No. 00/05704.However, the remnant voltage method can also be used with other types ofdisplays, for example active matrix displays which use an array oftransistors, at least one of which is associated with each pixel of thedisplay. Activating the gate line of a thin film transistor (TFT) usedin such an active matrix display connects the pixel electrode to thesource electrode. The remnant voltage is small compared to the gatevoltage (the absolute value of the remnant voltage typically does notexceed about 0.5 V), so the gate drive voltage will still turn the TFTon. The source line can then be electronically floated and connected toa MOS comparator, thus allowing reading the remnant voltage of eachindividual pixel of the active matrix display.

It should be noted that, although the remnant voltage on a pixel of anelectrophoretic display does closely correlate with the extent to whichthe current flow through that pixel has been DC balanced, zero remnantvoltage does not necessarily imply perfect DC balance. However, from thepractical point of view, this makes little difference, since it appearsto be the remnant voltage itself rather than the DC balance historywhich is responsible for the adverse effects noted herein.

It will readily be apparent to those skilled in the display art that,since the purpose of the remnant voltage method is to reduce remnantvoltage and DC imbalance, this method need not be applied on everyaddressing cycle of a display, provided it is applied with sufficientfrequency to prevent a long-term build-up of DC imbalance at aparticular pixel. For example, if the display is one which requires useof a “refresh” or “blanking” pulse at intervals, such that during therefresh or blanking pulse all of the pixels are driven to the samedisplay state, normally one of the extreme display states (or, morecommonly, all of the pixels are first driven to one extreme displaystate, and then to the other extreme display state), the remnant voltagemethod might be practiced only during the refresh or blanking pulses.

Although the remnant voltage method has primarily been described in itsapplication to encapsulated electrophoretic displays, this method may bealso be used with unencapsulated electrophoretic displays, and withother types of display, for example electrochromic displays, whichdisplay a remnant voltage.

From the foregoing description, it will be seen that the remnant voltagemethod provides a method for driving electrophoretic and otherelectro-optic displays which reduces the cost of the equipment needed toensure DC balancing of the pixels of the display, while providingincreasing display lifetime, operating window and long-term displayoptical performance.

As already indicated, a preferred type of electro-optic medium for usein present invention is an encapsulated particle-based electrophoreticmedium. Such electrophoretic media used in the methods of the presentinvention may employ the same components and manufacturing techniques asin the aforementioned E Ink and MIT patents and applications, to whichthe reader is referred for further information.

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the spirit and skill of the invention. Accordingly, the foregoingdescription is to be construed in an illustrative and not in alimitative sense.

The invention claimed is:
 1. A method for driving an electro-opticdisplay having at least one pixel capable of achieving any one of atleast four different gray levels including two extreme optical states,the method comprising: displaying a first image on the display; andrewriting the display to display a second image thereon, the methodpermitting any pixel to undergo at least two transitions withouttouching an extreme optical state, but being such that during therewriting of the display any pixel which has undergone a number oftransitions exceeding a predetermined value, the predetermined valuebeing at least two, without touching an extreme optical state, is drivento at least one extreme optical state before driving that pixel to itsfinal optical state in the second image, wherein the rewriting of thedisplay is effected such that, once a pixel has been driven from oneextreme optical state to a gray level, which is not one of the twoextreme optical states, in the direction of the other extreme opticalstate by a pulse of one polarity, the pixel does not receive a pulse ofthe opposed polarity until it has reached the opposed extreme opticalstate.
 2. A method for driving an electro-optic display having at leastone pixel capable of achieving any one of at least four different graylevels including two extreme optical states, the method comprising:displaying a first image on the display; and rewriting the display todisplay a second image thereon, the method permitting any pixel toundergo at least two transitions without touching an extreme opticalstate, but being such that during the rewriting of the display any pixelwhich has undergone a number of transitions exceeding a predeterminedvalue, the predetermined value being at least two, without touching anextreme optical state, is driven to at least one extreme optical statebefore driving that pixel to its final optical state in the secondimage, wherein said predetermined value is not greater than N/2, where Nis the total number of gray levels capable of being displayed by apixel.
 3. A method according to claim 2 wherein the rewriting of thedisplay is effected by applying to the or each pixel any one or more ofvoltages −V, 0 and +V.
 4. A method according to claim 2 wherein therewriting of the display is effected such that, for any series oftransitions undergone by a pixel, the integral of the applied voltagewith time is bounded.
 5. A method according to claim 2 wherein therewriting of the display is effected such that the impulse applied to apixel during a transition depends only upon the initial and final graylevels of that transition.